Legal protection_Question

Hello, there is unfortunately no legal topic forum here so I ask my question here if someone should have experience with it. My best friend sold his car 3 months ago. It is a BMW 320D BJ 2003 about 8500.– which had accident but was badly repaired. The buyer was informed during the sale and everything was recorded in the purchase contract. The buyer returns after 2 months and asserts that he has ekauft the car over the half of the value u nd either wants the money back or a price reduction of 4250.– Euro wants The friend unfortunately no longer has the purchase contract. Has something happened to you, what can he do there? He has no legal protection and is afraid that he has to go to court and event. everything can pay himself. Thank you very much.