Whatever you do, work it out clearly ahead of time!
Don't let an unspoken assumption guide you......you and your friend are likely to have different assumptions.

When I drive a friend's car in enduros, we have a pre-agreed max value on the car. If I destroy the thing, I give him the max value and keep the wreck (which I can sell back to him if he wants it).

If I am driving the car and damage it, I fix it (or pay to fix it, if I choose).

We don't have pefect definition of "destroyed" versus "heavy damage"....that's something we've talked about working out before next season.