Is it permissible for a man to return to his wife after she has been divorced by her first husband, if she married him without being divorced from the first and was unaware of the prohibition, and is their marriage valid?

The court is the most competent authority to consider these matters, as a judge's ruling is binding and resolves disputes. It is strange that the woman and her father would be unaware of the impermissibility of marrying a woman who is already married to another man; this excuse is unacceptable. Marrying a married woman is invalid and forbidden, in accordance with the Almighty’s saying: "And [also forbidden are] all married women" (An-Nisa: 24). Ibn Qudamah stated that whoever marries a married woman while knowing it is forbidden is an adulterer. If her husband divorces her and her iddah (waiting period) expires, then it becomes permissible to marry her.