1993 Honda Civic Problem with a friend and a shaky car.

I've had the car for 6 years.

I lent it last week to a friend to take a friend to the airport, several miles away on the freeway with some hills.

My timing is off, I think its supposed to be around 15 and it is at 11. (I think).

The next morning, I get in the car, start it and it shakes heavily when idle. It burns fuel, my gas mileage is greatly affected. It can't accelerate up hills well.

Ive never had any of these issues, he drives a 6cyl ford explorer (90's model I believe).

He claims he did nothing to it, my uncle, a mechanic, believes he had to have revved the engine pretty hard. As when exposing the number 2 spark plug there is a nic on the end of it. My question is, to get multiple opinions, do you think he is responsible for any damage the car has incurred? Or is it a coincidence the one time he drives my car this happens? I'm going to replace the spark plug as an inexpensive solution, but I have doubts this will solve the problem and I am worried my piston needs replacement.

I told my friend, as a heads up, that this could cost both of us some money. He sticks to his story that he only drove it, nothing more, and has become very upset. His roommate also mentions that he would say he has a lead foot (not proven, just observed).

Any help will be greatly appreciated.

Thank you.