Co-sign loan

My husand and his ex-wife, when they living together but not married, co-signed a loan for “his” daughter from a prior marriage. The daughter has stopped paying the loan, and despite our best efforts to get her to pay she won’t. His ex-wife is threatening to sue him to make him “pay off the loan so it is out of her name.” This “loan” was never part of a separation agreement having to do with the marital estate. It wasn’t even part of their own “settlement agreement” when they split the second time after living together. All she has is an email from him saying he would do his best to either get the daughter to pay it off or attempt to do it himself. He doesn’t have the means right now to “pay it in full”.

Does she have a case to take him to court over this? Isn’t she as liable for this loan as a co-signer as he is? How can she “force” him to pay this loan in full when there is no legal or signed agreement between them saying he would take full responsibility? Does this “email” count as an agreement or contract to pay? Thank you.

They are joint debtors on the loan. The email he sent would be admissible in court if she filed an action, but it is certainly not a promise to pay, all he says is essentially he will do what he can.

If they are both joint debtors then why is he more responsible than she is (under the law)?

He is not.