What should I do if I see someone doing something that I believe is forbidden based on the saying of a scholar, but I fear that there might be a legitimate difference of opinion on the matter that I am unaware of, and should I advise them in this situation?

One should not hasten to condemn something before verifying that there is no permissible difference of opinion regarding it. Munkarat (reprehensible acts) are of two types: first, the obvious ones that are known to the general Muslim populace; and second, those that are obscure or about which scholars have differed. No one should condemn except one who is knowledgeable about what he commands and forbids, and he must be gentle, just, and learned. Whoever acts without knowledge, what he corrupts is more than what he rectifies. Therefore, one should restrict counsel to what one knows and leave what one does not know.