Nothing worse than letting someone wreck your ride and you getting stuck with the repairs. Last time that happened (and I DO mean the last time), a friend had borrowed one of my bikes, and promptly plowed it into a tree because he was goofing around. First, I made sure he was all right - no broken bones or anything - and surveyed the damage to my bike. Then, while he was still laying on the ground holding his ribs, I reached down and pulled his wallet out of his rear pocket, took out the money it would cost to repair the bike back to the way it was, and tossed his wallet back down on the ground. I got cussed at pretty good - a good half of which I'd never even heard before - but he never asked to ride any of my bikes again ....
I have and I will put up with a ton of crap from some people, but when it comes to sticking me with the bill for something someone else screwed up, especially if it's something of mine that they screwed up .... that don't cut it with me.