I think we have a related question on this site, which I'll include when I find it.
There's a rather simple answer for your friend. The Talmud could be considered the "foundation" of halacha. Almost every later halachic work refers to the Talmud at some point. (I say "almost" in the sense that Ramba"m is a respected halachic authority but he is known, generally, for not citing his sources.) So, it is always better to view the source of something.
The other reason to study Talmud is because it is far more than a halachic book or journal. The argumentative style of the Talmud, I have found, is extremely helpful in teaching a person thinking skills and such skills are an incredibly important life skill as a person matures. Thus, it is best to start learning Talmud at a young age. However, even people who have begun studying Talmud in their 60's and 70's have benefitted from the argumentative thinking. I've personally seen this change in many people, and I'm sure there's enough research evidence to prove this, as well. (I'd be curious if anyone found that Talmudic study could hold off Altzheimers.)
As for your friend's saying, "it is just these people's opinions", no it is not just an "opinion". If you look at the structure of the Talmud, it's an analysis, not an opinion. Rarely, does the Talmud state a rabbi's view and leave it at that without explaining why he arrived at that view. And, inevitably, the reasoning is challenged, sometimes by a contradictory statement the same rabbi said elsewhere! So, it's definitely not just a person's opinion. It's a carefully derived analysis, which, at the end, may not even be "valid". That's why you would need to see the Talmud to see both what and why they arrived at the conclusion. You wouldn't get this from just the Mishnah.
There is nothing wrong with coming up with your own conclusions, and, in a sense, that's encouraged in Torah study. But, you cannot use your own conclusions to decide halachic matters unless your a rabbi. And, even within that, there are guidelines.