A: Your husband's ex-wife won't stick with the payment plan, so what makes you think you would ever collect from her? Besides, her making good isn't going to solve your bigger problem: what the unpaid bill is doing to your husband's credit.
At this point, he needs to work out repayment directly with the issuer. That may not sound fair, but your husband failed to check that all joint accounts were paid off in full and closed down as part of the divorce. He's legally responsible and, by making that one payment, he acknowledged as much in the eyes of the card company.
Even if your husband thought his ex was taking care of closing the account, he should have been checking his credit reports to ensure that any joint card accounts were shut down (everyone is entitled to one free annual report from all three credit bureaus by going to AnnualCreditReport.com or calling 877-322-8228). While your husband will be justifiably frustrated, cleaning up this mess will allow you both to concentrate on what matters most—your future together.