Is it obligatory to inform the woman of the truth about the man she married, that he drinks alcohol and commits unlawful acts with girls, knowing that she is unaware of his true nature? And does sin befall the informer if his informing leads to her divorce or problems among her relatives, especially with the uncertainty of his repentance?

If it becomes apparent to you that a man is committing transgressions, it is obligatory to advise him kindly, in the hope that he might repent. If he persists in his error, he can be threatened with informing someone who can deter him. The basic principle is to conceal his actions as long as he keeps his sin private, and there is no benefit in informing his wife of what he does. If you inform his wife without intending to cause mischief, and as a result she is divorced or problems arise among relatives, we hope that there is no sin upon you. To find out if he has repented, you can ask trustworthy people who know him.