Your wife may have a cause of action against her ex; however that does not relieve her responsibility to the joint account that she held with her ex. If she was joint signor on the account she is still personally responsible for the debt to the creditor.
She is responsible. However, this debt should not affect you. If the creditor gets a judgment against your wife they may initiate a variety of actions to collect, including garnishment of wages, accounts or taxes. They may get a lien on real property. They may file an execution against your wife's personal; property. They may repossess her automobile. Your joint accounts and joint property will be affected only as to the value SHE has in it. Your portion of accounts and property should not be at issue.
Please be advised that the above response does not constitute legal advice. The answer is provided for informational purposes only. You must make an appointment for a consultation with an attorney for an extensive review of your situation.