Rambam says in Melachim uMilchamot - Chapter 8 (11)
Anyone who accepts upon himself the fulfillment of these seven mitzvot and is precise in their observance is considered one of 'the pious among the gentiles' and will merit a share in the world to come.
This applies only when he accepts them and fulfills them because the Holy One, blessed be He, commanded them in the Torah and informed us through Moses, our teacher, that Noah's descendants had been commanded to fulfill them previously.
However, if he fulfills them out of intellectual conviction, he is not a resident alien, nor of 'the pious among the gentiles,' nor of their wise men.
The Rambam in Hilchot Melachim 10:9 writes:
The general principle governing these matters is: They are not to be allowed to originate a new religion or create mitzvot for themselves based on their own decisions. They may either become righteous converts and accept all the mitzvot or retain their statutes without adding or detracting from them.
(They are allowed to take on more Jewish mitzot with some exceptions as the Rambam continues and they must be performed accurately. This may lead to conversion).
According to Sefer Sheva Mitsvos Hashem vol I, Perek 3 (which deals with the prohibition of creating a new religion) they are forbidden both to add or subtract.
Regarding the concept that a non Jew can elevate sparks.
What is it to elevate sparks?
This is a kabbalistic/chassidic concept that there was an event figuratively called 'the shattering of the vessels' that caused sparks of light to explode out and leave the realm of kedusha (holiness) and falling into the four impure husks of the sitra achra (other side).
I would contend that while well intentioned the Rabbi was not quite accurate, although there is one situation he may be right.
Tanya (seminal work of Chassidus) explains that a Jew has two souls. Chapter 2, Likutey Amarim
The second, godly soul is what elevates sparks.
Out of the four impure husks, there is one that is neither inherently good or bad. It is intermediate. It is called klipas nogah.
All Noahides come from klipas nogah.
All converts come from klipas nogah.
As a person ascends closer to kedusha, to the point they may cross over into kedusha through conversion, they become like a high priest according to their place.
Nevertheless they are still on the "other side." Therefore any sparks consumed there cannot be elevated to kedusha unless that person eventually converts. As the food one eats becomes a part of them the eventually conversion in a sense elevates these sparks as the person will be using that body to serve HaShem.
Yet there is a story in the book 'Bnei Avraham Ahuvecha: Gerim in Chassidic Thought (Dov ben Avraham, the Breslov Institute) about a convert who was put to death as a martyr, who had rejoices in his Jewish soul but explained how he had lamented over his goyish body and desire to be freed from it. Sometimes the past leaves a residue that can be hard to overcome. Those traif foods dull the mind and make it harder to learn.
Rebbe Nachman of Breslov said,
"Although a ger has a body that is not very holy, he has a soul that comes from a very lofty place. The ger must focus on the final outcome, his soul, and not on the fact he was formed from an impure drop of seed." Likutey Moharan II:33
"The soul of a Jew is a "portion of God above." The holy books explain that this is an actual portion. Once a ger completes his geirus, he separates the life force of his body and soul from the Other Side and attaches it to God." (Likutey Moharan v'Toldos Levi ben Yitzchok. Megillas Rus)
By definition, it is the job of a Jew specifically to elevate sparks which means to extract the good trapped within the other side, and incorporate it into oneself, thus elevating it to kedusha, where the godly soul of a Jew exists.
"The Sages of the Talmud taught that prospective gerim (converts) will not be accepted in the days of the Messiah. One reason for this is connected to the statement that, unlike the Egyltian exile, there will be no need for thr Jewish people to leave this Exile in haste - this is because all the sparks of holiness that the Jews are in Exile to elevate will have been elevated.
When in Egypt, the Jewish people were commanded to leave in great haste as soon as all the sparks there had been elevated, and to travel to other locations to continue this work. However, once the last spark in thr world is elevated, there will be no place else for the Jewish people to hurry to, and they will then leave Exile calmly and peacefully." (Kesser Torah, Parshas Vayigash, cited on p. 53 of Bnei Avraham Ahuvecha: Gerim in Chassidic Thought).
One more thing, the Lubavitcher Rebbe said, "Bring me lights of chaos in vessels of rectification."
The lights of chaos have to do with Edom and the sitra achra while vessels of rectification refer to Torah.
Certainly the only reason for the continued existence of the false religions is only because of the existence of holy sparks entrapped within them. Mundane ideas that are neither fundamentally good nor bad but have a holy use within the framework of Judaism.
It is certainly not the case that these religions are kosher or one can whitewash something with a fundamentally corrupt source, but one of the reasons for delayed geulah (redemption) is to give us the time to extract all the remaining good trapped in the world that can be used in the Holy "vessels" of Torah. Light that can be contained without causing another shattering event.
When all those sparks are extract, by Jews, both born and converts, then there will be peace on earth and everyone will know and understand. The nations will say, "We have inherited lies" (See Jeremiah 6:19-21).