What ever you do -
don't feel responsible! It's entirely possible that this guy was yahooing around and over revved it and it flew apart. If you knew it was in good condition and were not aware of any problems it's caveat emptor.
I once sold a 250 cc dirt bike to a guy, I had rebuilt the engine and had about 4 hrs on it so I knew it was good. He called me 2 hrs after he picked it up and said it just stopped. I felt bad for the guy and bought it back (hadn't had time to go to the bank yet!)
When I pulled it apart the rod was broken and blue. He either didn't get the mix right or he over revved it, or both.
Cost me several hundred in parts and gaskets to get it running again.
Then I rode it for another 4 hrs and never had a problem.
GET THIS, the next guy that bought it calls me
3 months after he bought it to say that the front forks weren't original, the back wheel wasn't original and the seat had been recovered, HE WANTED HIS MONEY BACK. Are you kidding me??? It was a 25 year old dirt bike!!!! What's going to be original?
My point in these stories?
If you looked after it and sold it in good faith, and he broke it.....tough luck for him. Don't feel bad because there's a lot of people out there who are looking for someone gullable to pay for their mistakes.
OK rant is over. Whew, I needed that, now I gotta go play halo and kill something.