Can a debt collector sue my husband for his ex-wife's auto loan for which he cosigned

(Texas)

My husband cosigned for an auto loan for his ex wife in 1996. She stopped making payments in 2000 after the divorce and it was repossessed and sold at an auto auction. I just received a call (Sept. 2011) that they were trying to collect the amount not covered by the sale, $7,000 plus interest since then, another $6,000+. They've tried reaching the ex but since they couldn't they're threatening to sue my husband. Can they do this? What options do we have? Thanks