Is it obligatory to inform the husband of what my colleague did—committing adultery with his wife—or is it obligatory to conceal their sin?

If a man confesses to committing adultery, he has erred in two ways: committing adultery, which is a major sin, and openly declaring it, which is another sin. It is even worse if the adultery was with a married woman. He should have repented and concealed his sin. He must be advised and warned about God's punishment, and if he does not repent, he can be threatened with exposure. As for the woman, if she persists in her wrongdoing, she can be threatened with informing her husband, and if it becomes necessary to inform her husband, it should be done subtly so that he may pay attention to his household.