Is it obligatory to inform the one who broke his oath of his breach, and what if informing him would lead to harm?

If you know that a man has taken a binding oath (yamin mun'aqidah) and has not offered expiation (kaffarah), you must advise him by explaining the expiation required of him. Neglecting expiation is an evil that must be rectified according to one's ability. If one cannot do so with their hand, then with their tongue; and if not with their tongue, then with their heart, and that is the weakest of faith. However, if the man is ill-tempered and does not accept advice, or if advising him would lead to a greater harm, then there is no blame upon you for remaining silent. Your disapproval with your heart will suffice. The purpose of enjoining good and forbidding evil is to reform people, not to merely satisfy one's indignation.