If you are a group, then the group pays for it. Yes the driver was at fault, but out of the 6 of you, how many would have done the same thing? It stinks no matter how you slice it, but the bottom line is the trailer was borrowed, and if you want to use it again, I would suggest everyone shell out some cash, get it fixed and not make the fellow suffer who was nice enough to loan you all the trailer. That said, sounds like the fellow driving made a mistake, which we all do, I would also suggest having some sort of agreement in the future to cover this type of situation.