what if the separated wife won't sign check?

My son and his wife have been separated repeatedly. She kept the car in question that was totalled by her father. The insurance has settled and now requires both signatures and the bank will not deposit with out them. We've tried to do a deposit only by myself - they would not accept even after telling me on the phone they would. She is pretty much refusing to sign it because she is obviously wanting the check. They are both strong willed and I've suggested splitting it but theres no give between them. I understand that my son paid the majority of all payments even when separated, but she traded her vehicle in on it when they got it. There is so much to the story but we just basically need to know what to do if she will not sign!