How to Address Ex-Spouses Delinquency on a Car Loan

My husband co-signed for a car for his now ex-wife and during the divorce, it was determined that she would keep the car and be responsible for the payments. This has been over four years ago and he has received many notices regarding late payments. He said he contacted the lender and notified them of the divorce and provided the decree but he continues to get calls and notices on anything related to car including parking tickets. This has really impacted his credit. What actions does he need to take to stop these negative inquiries on his credit.