Check with the insurance company and see what it would take for them to send you copies of the explanation of benefits reports from medical services. From a credit standpoint you're better off paying the debts and then seeking reimbursement from the ex-. However, although this depends upon the full facts and the laws of your state, you may also have the defense that it was the mother who signed as the party who agreed to cover uninsured expenses, not you, and can try asserting to the collection agency that they don't have a contractual basis to hold you responsible for the copay.