My question involves collection proceedings in the State of: TN

My elderly folks co-signed on a personal 45K loan with my sister and now ex-brother-in-law 4 years ago. The ex-brother-in-law stopped paying on the loan after about 1 year - and my poor folks have had to pay on it since then. How is the best way to legally get my ex-brother-in-law to pay on this ? Any guidance would be appriciated!