Is it permissible to assist a non-Muslim in sin, or to advise him to avoid it, or to leave him alone because he is a non-Muslim?

Aiding in disobedience is forbidden, and aiding in abandoning it is obligatory, due to the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression."

A Muslim is commanded to choose righteous companionship, and should only associate with believers, due to the Prophet's (peace and blessings be upon him) saying: "A person is upon the religion of his friend, so let one of you look at whom he befriends," and "Do not associate except with a believer, and let none eat your food except a pious person."

Bad companionship is harmful in this world and the Hereafter, due to the Almighty's saying: "And the Day the wrongdoer will bite on his hands, he will say, 'Oh, I wish I had taken with the Messenger a way. Oh, woe to me! I wish I had not taken so-and-so as a friend.'"

Sitting with people who commit evil is forbidden, and whoever remains silent about their evil is like them, due to the Almighty's saying: "Indeed, you would then be like them."

It is obligatory to avoid associating with wicked people and to seek righteous companionship. If it is not found, then isolation is safer, and Abu Dharr's saying: "Solitude is better than a bad companion."

Advising non-Muslims is not entirely without benefit, especially regarding sins that are unanimously forbidden. More important is inviting them to Islam.