Is it obligatory for a man to forbid evil to a woman who is not one of his mahram (relatives one may not marry)?

The Shaykh : Yes, indeed. In any case, I do not think anyone who is not in a position of authority should do so. Otherwise, if a man is with her, he should speak to the man. Because if he forbids her directly, she will get into a difficult situation; she will scream and say, "I am one of your women, who am I, one of your quarters?" Then she will smear you with a calamity, saying, "This man is flirting with me." Yes, this is what actually happens.