What is the duty of a man who has repeatedly engaged in sexual intercourse with his wife's mother out of a relationship of love and passion, and then realized that he is committing a great sin?

Adultery with the mother of one's wife is among the gravest sins, and its enormity is greater than adultery with a strange woman. If a legal ruling were to be found, both the perpetrator and the one with whom the act was committed would deserve execution. Those who committed this abomination must repent to Allah Almighty and sever this forbidden relationship. They should treat each other as strangers; private seclusion, handshakes, or conversation are not permissible except within what is known to be proper. The woman must observe hijab in his presence, for jurists have stipulated that a mahram becomes like a stranger if temptation is feared. And whoever repents, Allah will accept his repentance, as Allah Almighty says: ﴿Except for those who repent, believe, and do righteous deeds. For them, Allah will change their evil deeds into good deeds. And Allah is ever Forgiving, Most Merciful.﴾