What is the ruling on a man who had sexual intercourse with his wife's mother: Is he obliged to separate from his wife for life, and does she become unlawful for him even if he divorces her and then takes her back?

Whoever commits adultery has committed a grave indecency. The matter becomes even more heinous if it is with a forbidden relative (mahram). The adulterer must hasten to repentance and seek forgiveness. The wife does not become forbidden to the man if he commits adultery with her mother, according to most scholars, and he is not required to divorce her. If he divorces her, he may take her back before her waiting period (iddah) is complete, or renew the marriage contract with her after her waiting period ends, as long as he has not divorced her three times.