Is it permissible to force a husband to divorce his unbelieving wife, whom he married through an 'urfi contract?

If the wife is not from the People of the Book, the marriage contract is null and void without dispute. If she is from the People of the Book and chaste, then his marriage to her is permissible. An urfi marriage, if it fulfills the conditions of the guardian, witnesses, offer, and acceptance, is valid according to Islamic law. However, if it is without a guardian, it is invalid according to the majority of scholars. If the marriage is invalid, you must advise him to separate from her. If he does not comply, then refer his case to the judiciary. But if his marriage is valid, you are not permitted to demand that he divorce her merely because she is your co-wife.