Is it obligatory to advise someone whose thigh is exposed, and is one sinful if they do not do so?

One should not condemn a person who commits an act over which there is a difference of opinion, if he believes it to be permissible. However, if he believes it to be forbidden, then he must be condemned. If he does not believe it to be permissible nor forbidden, then it is preferable to guide him without reproaching him. Al-Izz ibn Abd al-Salam mentioned that if someone commits an act whose prohibition is a matter of difference, while believing it to be forbidden, then he must be condemned. But if he believes it to be permissible, then it is not permissible to condemn him, unless the basis for his permissibility is extremely weak. If he does not believe it to be forbidden nor permissible, then he should be advised to avoid it.