My sister cosigned for her now ex husband a car and he got it repoed...?

Now they are calling her for the money they didn't get when they sold it. The car still had almost 17 thousand owed on it and they only sold it for 9grand. They are calling her and she is about to have a nervous breakdown over it. She has always had great credit and is very responsible. She teaches at the university and middle school, but was recently laid off due to low enrollment. Her ex is a s bag now. He let the car get repoed and went out and got a new apartment and car. He is the primary signer on this loan. Can she do anything about this?? She only signed because they were married. Now she is jobless and thinking about selling her car to try and pay these people when it is her ex husbands debt really. Any advice? She's already paid off all the credit card debt he ran up. He rarely pays child support on time either. Only when she starts callin him. Advice would be great. I hate to see her like this. Thanks.