What can we do if my husband’s ex wife is filing a lawsuit for back child support?

First, your husband screwed up by not paying through the state. The judge may or may not believe him about what he paid, more than likely not because there is no paper proof, except he should get her bank records to see if there were deposits that would have come from him. Second, he should ask for her to be held in contempt for not letting him see his child in accordance with the present child support order. Third, he should contact the police about her filing false applications for credit.

The problem with cash and money orders is it's tough to prove payment. First thing he should do is establish a checking account to prevent this from happening in the future. He can request copies of money orders if he remembers the company he used. He can request copies of her bank records and if it shows deposits in the amount of his cash or money order that might help. But he could be on the hook to pay again. The credit card stuff has nothing to do with the issue.

To be painfully honest with you, if he paid child support in cash and his ex-wife won't admit it, AND he has no way of proving (with receipts) that he paid he, he's screwed. As for credit cards she opened in his name, you *can* do something about that, but I wouldn't use the fraudulent credit card accounts as a defense to the child support claims. The fraudulent credit card accounts would be a separate matter. Once you get a judgment against her, I would then argue that they offset what he will end up owing her. Now don't be a tightwad and go meet with a good (not just any) divorce attorney for help. Yes, it'll cost you, but it's money well-invested to ensure you don't worsen your situation by staying ignorant of the law and/or vulnerable to a legal system that isn't kind to men, especially men accused of being deadbeat dads by ex-wives who have no compunction against lying and committing fraud. For the love of humanity, get a good lawyer.