Legal Question in Real Estate Law in California

I never heard of "wrong gas" necessitating repairs. Did this happen over an extended period of time, or was the substitution diesel for gasoline or vice-versa?

I'd say if the car were "borrowed" without your knowledge or permission, the friend is liable for whatever happens. If you lent him or her the car, it would probably depends upon whether or not you gave clear and explicit instructions that the friend negligently failed to follow. If the friend's negligence caused the need for repairs, the friend should pay. Otherwise, it's your own responsibility.

Still, some mechanically-competent explanation is needed for the connection between the choice of fuel and the need for repairs. This is a bit unusual.

If you have proof of damage, and proof he caused it by mistake and negligence, you can sue if he won't pay. You'll need clear testimony from automotive repair experts, not just your word. "Wrong gas", probably wouldn't damage an engine without additional abuse and negligence; gasoline in a diesel would - quite expensively.