Same crap happened to me once. Sold my wife’s car and the lady called 2hrs later said the ride was bad and she has chrones disease and its painful to drive. Called my buddy who is a mechanic as I was totally stressed. He gave me the best advice. He said do NOT give her the money back. You have no idea what this person did to the car. Could have drove over a curb and screwed up the steering or even worse. I told her that and she said ok and I never heard from her again.
My opinion, say the same to this guy and he will stop bothering you.
Best of luck.