Siman 303

The Rambam wrote at the end of chapter 10: One who sees kilayim of the Torah on his fellow, even if he was walking in the marketplace, jumps toward him and tears it off him immediately, and even if it was his teacher who taught him wisdom, for human dignity does not override a negative commandment explicitly stated in the Torah. And why is it overridden regarding returning a lost object? Because it is a matter of money. And why is it overridden regarding the impurity of a corpse? Since the verse specified "and for his sister," they learned from the oral tradition: for his sister he may not become impure, but he may become impure for a met mitzvah. But a matter whose prohibition is from the words of the sages, it is indeed overridden by human dignity in every place. And even though it is written in the Torah, "you shall not deviate from the matter" (see Sefer HaMitzvot, Root 2), this negative commandment is overridden by human dignity. Therefore, if there was shaatnez of the words of the sages upon him, he does not tear it off him in the marketplace and does not remove it in the marketplace until he reaches his house; but if it was of the Torah, he removes it immediately. Thus far his words.

From his words it is clarified that he possessed the version in Berakhot (19b) "one who finds shaatnez in the garment of his fellow" and not "in his own garment" like our version, and therefore even though the wearer is unintentional, nevertheless one is obligated to tell him. The Tur in the name of the Rosh disagrees with him, for specifically when the wearer finds it himself, which is intentional, but not one who sees it on his fellow while the wearer does not know and is unintentional—he should remain silent because of human dignity, see there, for this is called "sit and do not act" which was permitted for the sake of human dignity as we say there. The Rambam holds that this is called "arise and act" since he knows of it. Our teacher, the Rema, brought the opinion of the Tur, see there. However, the Rambam certainly admits that regarding a rabbinic prohibition, even in his own garment when he sees it, he does not need to remove it in the marketplace until he comes to his house. In the Yerushalmi it is implied like the opinion of the Rosh. One should not learn from this to other prohibitions where Reuven knows that Shimon is transgressing a Torah prohibition and Shimon is unintentional in the matter, that he does not need to tell him; for it is different here as it is for a brief moment, for when he comes to his house he will tell him and he will strip it off, but regarding a permanent prohibition one is obligated to tell him (Noda BiYehuda, siman 35). And even regarding a permanent rabbinic prohibition, it appears to me that he is obligated to tell him and should not refrain on account of human dignity. And so too, when one is sitting in the beit midrash and sees rabbinic shaatnez upon himself, he does not need to hasten to leave, but not so regarding Torah shaatnez, and see below, siman 372.