Someone is trying to sue, help!?

My GF's EX is trying to sue because he was a co-signer on a purchase of around $1,500 - $2,000 ..She was unable to pay off her debt which is around $10,000 probably. Since he was the co-signer it fell back to him and he couldn't or wouldn't pay it so it ruined his credit as well. That's his case I guess. Here's the thing. Before they split ..He quit his job, was bumming off of her, sold their belongings for drugs ..drained their bank account, stole her cash and he's been stalking her/us for 3 years now, haring her Mother and he has been jailed before, DUI's, possession of drugs etc. etc. ..he's been living with his parents ever since the breakup with no job and he wants to sue. QUESTION: Is it possible? Would we even have to show up for court? Can she counter sue? How big of a "problem" could he cause? I think he's using this just trying to locate us. He seems obsessed.