What is the ruling on a Muslim man marrying a woman of the People of the Book who is 57 years old, 13 years his senior, and is known to have committed adultery and conceived out of wedlock? Is a marriage of convenience (business) permissible? And is justice required in cohabitation between two wives, despite their differing capabilities?

Your husband's marriage to this Christian woman is fundamentally invalid because Islam stipulates the chastity of a Kitābī (People of the Book) woman when marrying her, based on the Almighty's saying: "And the chaste women from among the believers and the chaste women from among those who were given the Scripture before you, when you have given them their due dowries" (Al-Ma'idah: 5). The term "chaste women" here refers to the virtuous and modest, as interpreted by Al-Sha'bi. It is also narrated that Umar ibn Al-Khattab (may Allah be pleased with him) ordered Hudhayfah to divorce his Jewish wife for fear of marrying unchaste women from among them.

Marriage to an adulteress is forbidden, whether she is Muslim or non-Muslim. Allah has explicitly stated this in Surat An-Noor and has forbidden it for believers. Imam Ahmad issued a fatwa prohibiting a chaste man from marrying an unchaste woman until she repents, and similarly, a chaste, free woman from marrying a corrupt man until he repents.

Therefore, your husband must annul this marriage and repent to Allah. You should deal with him wisely and explain the seriousness of his action, seeking help from wise and righteous family members to advise him.