Dude broke your stuff, plain and simple.
Any respectable bassist would have, at the very least, not thrashed your bass and broken your strings in the first place. Failing that, they should've approached you, apologized for it and offered to pay for them (an entire set). I mean realistically, if the set had been on for very long, the remaining strings will likely sound very different from the new ones and that just won't do. I might see it as an opportunity to teach someone a lesson. If the chap was any bright at all, he'd have already been treating your baby gingerly and if he was
real smart he'd of realized what strings he was playing on and not been quite so abusive. Still, people never cease to amaze me with their infinite stupidity so I try not to give them an inch to start with.
Before I let people play one of my instruments I make sure that they understand and agree to my conditions. I don't have to do this very often anymore because I'm no longer a house bassist for a jam session. Back then, I'd bring a sturdy 4-banger for bassists to use since I wasn't entirely comfortable with them playing my 5 or 6 string and most of them preferred a 4 anyway. Certainly, folks that I trust or know to be of good character, are welcome to play anything they like, still acknowledging the rules, but I can't stand the random jerk that comes and demolishes an instrument to show off to his friends or for his own amusement. That sort of absent-minded abuse is too-often dismissed and I don't put up with it.
Most of my gigs these days consist of several bands playing one hour sets throughout the night. Nobody really shares equipment, save for the house gear of course. Everyone brings their own instruments, their own cables, etc. We don't often have rhythm section players sit in so there's not much of that though those that do sit in are trustworthy and wouldn't hesitate to pay for a string or a set had they broken one or several. Regardless of whether the person is a total stranger or trusted friend, they should take full responsibility for their actions and reimburse you fully not only for the two strings broken but also possibly for a whole new set. That's up to you. I don't see why you should have to be overly-nice about it. Something along the lines of, "I'll bet you didn't realize how expensive those strings were, did you?" Maybe you should teach him a lesson?
Who slaps on flats these days anyways?

I know someone's gonna say they do it all the time and it sounds great but I'm just sayin'. Not the greatest noise on the earth. What's that funny quote from Tony Levin's book
Beyond The Bass Clef? "It also sounded something like the movement of furniture, which He hadn't even created yet, and He was not so pleased. And He spoke to the man, saying "Don't do that!"
