Someone put something in my gas tank without my consent- can I sue?

I had my 2000 ford f350 7.3L diesel (with 70K original miles) for sale. A man from California flew to Houston (my home) to purchase. During his review, we went to AutoZone so he could have an employee run the free diagnostic report. Everything came back normal, no lights were on, etc... The man purchasing the vehicle bought a big bottle of diesel kleen and added it to the fuel tank.. I didn't realize he had done this until he had finished (I could smell it). Before even going to AutoZone I mentioned that I filled up the tank with diesel and put some DieselKleen in. He added a ton more "To improve performance".. On the way home, the truck stalled and died multiple times. I told him it must have been running ultra rich from the extra injector cleaner. Long story short: He purchased the truck. 2 hours later he calls saying the check engine light was on. The next day he said he drove it back to California and it barely made it there. He was able to get it to the ford dealer who said it needs a whole new motor. He says I have two choices: Pay for the new motor or refund his money. I have NEVER had a problem with the motor nor seen the check engine light on. It wasn't until he, without my knowledge and consent, added something to my vehicle (before even finalizing the purchase) when the problem occurred... WHAT CAN/SHOULD I DO????