If one is unsure if one can be yotze kiddush through someone, i.e. he may be mechallel shabbos b’tzina or doesn’t believe fully in one of the yud gimmel ikkrim and one cannot make kiddush for oneself as this would embarrass the person, can one say the kiddush quietly along with the person (so that the person doesn’t realize) and then quickly drink from the wine cup at the conclusion of kiddush after the person drinks?
It is permitted to do so.
However, this is not lechatchilah, and should only be done if you have substantial cause to believe that the person making kiddush is not qualified to do so. The fact that he might be violate the Shabbos privately, for instance, will not be sufficient cause, for even if he does so this will not disqualify him.
See Shulchan Aruch, Orach Chaim 183:4, concerning the cup of wine for benching, and the same principles apply for Kiddush, as ruled by the Shulchan Aruch (271:10). The Mishnah Berurah writes that holding the cup does not disqualify the Kiddush on a bedieved level (271:78).