We did have a similar experience and no, I would not expect them to pay. I can't help but feel that a lot of this has to do with the fact that you don't like these people and don't want your son to be friends. My kid has been to ER three times with broken bones. All happened on a property not owned by us and I never felt the cash should be coming from somebody else's pocket. Two of those breaks involved surgery and he was not down for long. Would you be going through so much angst if it happened at your best friends house?