The 'Mean Boy'

OK, so there's a couple in town that often invites is to do stuff with them. We really like them, but their 5 year old boy is really mean to our 5 year old girl (she calls him "the mean boy"). I have seen first hand that this boy really is deliberately mean just for kicks, but his parents don't seem to notice or don't seem to be able to do anything about it. Meanwhile, when they invite us over, my daughter freaks out of course and doesn't want to go to the mean boy's house. It doesn't seem right to force our daughter to go over there, but how can we tell them their kid is a nasty SOB? Not sure how to deal with the situation. What do you guys think?