Sued after over 13 years?

I have a buddy of mine at work. When he was 18 years old he bought a car for $12000. The financing company told him that he missed a payment when he didn’t. Him being young he paid the supposed missed payment. After about 5 months they contacted him again saying he missed yet another payment when he didn’t. They threatened to repo it and he told them to do it. They auctioned the car for $4000 and told him to pay the difference. By the time he was 25 years old he finished paying off the $4000 that they settled with him on instead of the $8000 for the difference. They even took it off of him credit report when he was 25. So here he is, 6 years after it came off him credit report, and 13 years later, and he gets a call from a lawer hired by the financing company to sue him for $12000 that he owed on the car. Saying he never paid a dime. Does anyone have any plans of options I could give him? He lives in Georgia at the moment if that helps.
On top of that, he’s received nothing in writing from the law office or the finance company stating that he’s to be sent to court. He’s only been contacted by phone and they’ve threatened to send him to court if he doesn’t have proof that he paid it off within 24 hours. They read him his social security number, and every address he’s ever lived at. He said it sounded fishy because when they called him initially they stated that they read it from his credit report and it was removed when he was 25. Sound fishy to anyone else? What should he do?
New Info. He gave me the phone number to the “law firm.” It turns out that they’re not in california at all. They’re a collection agency in Washinton called Green Tree Acquisitions. They’ve broken several laws thus far such as harrassment, calling of family members, raising their voice, not informing him that they’re a debt collection agency, threatening his credit, and this is only the peak of the list. No he doesn’t have any receipts, like I said, he’s 31 now, this all happened when he was 18 and was settled at 25, after 13 years he assumed it was over and just recently tossed the paperwork. I also learned that they couldn’t even sue him if they wanted to due to the Statue of Limitations Laws.