My friend does not practice Rabbinical Laws unless he fully agrees with the reasoning.
He's totally fine with direct Torah Laws however.
I'm running out of arguments to convince him. Any ideas?
Here are some thoughts. I'm not familiar with the process of Jewish law (I'm not Jewish), so I'm not writing with authority. These are just answers to questions that I've asked in my own desire to understand where I can hear that God is speaking.
Questioning rabbinical law
In order to find some common ground, it would be helpful to listen to what his real concerns and reasons are. How did he come to this position, and why does he find it important to stay there? I would especially wonder how he knows what reasoning to apply to the written Torah without taking part in the wider rabbinic conversation and its traditions regarding that process. But there may also be some very important issues to consider from his point of view.
Judaism is a revealed religion and the commandments you follow are from God, so you can't just put words or commandments into His mouth. Sometimes your integrity before Him in prayer is directly at stake, as in brachot where you thank Him for giving particular mitzvot. There's also the objection that rabbinic law is much more detailed than the written Torah, so that sometimes a mountain of halacha is hung from a single thread from Torah; and that the Pharisaic approach to Judaism isn't reflected clearly in the earlier records of Tanach. The question arises about how much is new. Historically, why should a Jew believe that much of the oral Torah can be directly traced to the community in Moses' time, or that until the last part of the Second Temple period there was hardly any disagreement between rabbis? And how can you separate the commandments of God from human customs and traditions that are valid but not binding, when the two are presented to you inseparably? What about the issues of social dynamics, controlling attitudes from leadership, and the willingness of some crowds to follow like sheep? Or the fact that some decisions by trusted Jewish leaders are wrong, and it can take a long time for these things to become clearer to the community? How do you keep your personal responsibility in the short term?
Your friend needs to understand that God has preserved His words and a testimony about Himself, and His Law, among all the generations of Israel, but the reality is that not just any group can claim that they uniquely hold such truth and authority. Even among diverse groups who recognise each other's legitimacy but hold carefully to their own rulings, the differences might bring unnecessary divisions. And because of the many fences that have been instituted to protect and preserve both the letter and the spirit of Torah amidst Israel, it needs to be known that every stringency inherently holds a leniency, at least in the ability to be close to the reality of Torah and to have the peace of mind that is needed for following God according to the holiness and blessing of this covenant.
Reasons why rabbinic law is binding
That said, I don't believe that Jews can keep the Torah truly and completely without doing so in accordance with community, honouring the huge conversation of Judaism about how to keep and honour Torah, and submitting to the things that are agreed upon in the Talmud. As one who can't comment too much, I still believe that it's important in light of how God created His relationship with you as a nation, together, and the particular role that you have in His program for humanity. Here are some reasons.
In my observation, it seems like the tradition is most commonly ignored when the truth and original intention of the written Torah is doubted as well. It's so clear in Tanach and in our own experiences that God has given a unique kind of witness to Israel as a nation, in collective observance and preserving of the Torah. You are kept by the Torah and you keep pointing back to it, to the knowledge of God in our world. It's true that aspects of Judaism in each generation are wrong and need to be corrected; it's also true that such correction can take time to spread throughout Judaism, and that the discussions, arguments, and decisions can be painfully divided and complex. But in the generations before us and in our own generation, every Jew will find that there are others around them who have so much deeper familiarity with the spirit and details of the Law, and greater love and dedication for the Law, than their own. And finally, there's the quality of being able to make decisions concerning the law on the level of the eternal community, weighing out the ramifications of your decisions across the generations: a quality which each individual alone possesses none of.
Firstly, I don't know that you necessarily can convince your friend, but he might wish to consider the insufficiency of the Torah without any form of additional, correlative instruction. For example, how might he deal with contradictions within the text? How might he deal with passages that go into insufficient detail for their laws to be put into practice? And how might he deal with passages so horrendously barbaric that they are literally crying out to be expounded?
For some specific examples, consider the following three:
• Exodus 12:5 makes it clear that one may only use a lamb or a goat as the Pesach sacrifice, but Deuteronomy 16:2 allows one to also sacrifice cattle. The rabbis understood the second of these passages to be referring to the Chagigah instead (Sifrei §171, Mishna Menachot 7:6 [82a], etc). How does your friend reconcile these passages without recourse to the Oral Law?
• Deuteronomy 12:20-21 allows for the slaughter of animals in order to eat meat, but demands that they be slaughtered in the manner "that I have shown you". Given that nowhere else within the Torah is that method even alluded to, what might it have been? As the Rambam points out (Hilkhot Shechitah 1:4), the specific practices that we follow are entirely rabbinic - which is to say, entirely expounded in the rabbinic literature. How does your friend construe the passage from Deuteronomy without recourse to the rabbinic tradition?
• Deuteronomy 21:18-21 mandates of parents that they take their stubborn and rebellious son outside of the city and stone him for his gluttony. The Mishna (Sanhedrin 8) goes into great detail, interpreting this passage in such a fashion that it cannot ever be put into practice, without at all abrogating the actual law that it describes. How does your friend, without recourse to such a tradition, see fit to nullify or explain this particular law away?
These are only three examples; there are abundantly more for all three types of problems. But be aware: none of these arguments specifically justifies the rabbinic tradition. They speak to the fact that there absolutely has to be something more than just the written Torah, if your intentions are to live your life (and moreso, run a state) in accordance with its principles. Whether or not your friend chooses to embrace the idea that the rabbinic traditions are from Sinai, he must reckon with the fact that the written Torah alone is insufficient, and with the fact that all groups who have argued in favour of its sufficiency have, in time, developed their own correlative legislation anyway.
Here is another source: In Deut. 12:21, it says "Then [when Israel enters the Land] you may slaughter of your herd and flock . . . as I have commanded you." Ask your friend to find the commandment described in the verse. It cannot be found, this was one of the instructions on performing the commandments that was given ORALLY to Moses.
Also, he might refer to the story at Num. 15:32-36, where a man was found gathering sticks on the Sabbath. The congregation all knew he had violated the Sabbath. But where, in the Torah, is there a commandment about gathering sticks? It says you shall do no work, but the Torah really doesn't define the term. And yet, the people all knew that part of the law (they didn't know the method of execution though), and knew that the law-breaker also should have known better. Where did they hear the details? Moses got it from the Oral law given by Hashem.
However, these verses only help define those laws which the rabbis have concluded came directly from G-d in an oral tradition. It does not explain their authority to enact rules to help us avoid violating Torah laws. However, at Deut. 30:16, we have a Biblical commandment to keep G-d's commandments, statutes and ordinances. How are commandments, statutes and ordinances different. Commandments refers to the easily-found "Thou shalt..." commandments in the Torah. The statutes would refer to the Orally-transmitted laws such as I referenced above, or those we learned from the Naviim (prophets), such as the rule that no kohen can marry a convert. That leaves ordinances. These, I would submit (and -- others can help with a good citation I can't find right now -- I believe the rabbis agree) are the rabbinic rules necessary to apply Torah to an ever changing world. Their authority comes from Deut. 17:11 ("According to the law which they shall teach you, and according to the judgment which they shall tell you, you shall not turn aside from the sentence, which they shall declare to you, to the right hand nor to the left").
You might also want to check out Moshe Koppel's 1997 book "Meta-Halacha: Logic, Intuition and the Unfolding of Jewish Law (Jason Aronson 1997) which whether rabbinic law is intuitive or Biblically based. I have not personally read it, but my Rav's father-in-law was giving me a review of it this past Shabbos.