What can I do if my ex father-in-law is suing me for money he lend me?

While we were engaged, my ex-wife and ex-father-in-law convinced me to borrow money from him to pay off two credit cards 6 months prior to the wedding. A large portion of the credit card debt was from purchasing the engagement ring. She agreed that we would pay the loan back together once we were married. While married, we paid monthly payments to him out of our joint checking account. Two years later she filed for divorce and refused to return the engagement ring, stating that it was a gift. Once she filed for divorce, we stopped paying back the debt to her father. Now, almost a year after finalizing the divorce, he is suing me in small claims court (Illinois) for the remaining debt of $4,500. The ring was purchased for $4, 484 and valued at $6,000. I still have the receipt from the ring and a statement of its value. He has no promissory note, only copies of the signed checks and a repayment history. Does my ex-father-in-law have a case? Should I involve my ex-wife and try to put half the debt on her since we agreed to pay it back together? Also, our divorce papers state that each of us is responsible for any debt we incurred separately before getting married. Is it possible that they will get away with my $6,000 ring plus my $4,500?