If your minor child gets hurt at my house, you can sue me for negligence. If I didn’t talk to you beforehand, you’ve got a much easier case. If I then present evidence that you were abusive, you’ve got a much harder case. You’ve got to feel out the situation a little bit, but currently there’s no indication that her parents are crazy. They could tell their son that they’ll need to talk to the parents first and see how he responds.
I like tequila with a pickleback, which is the way less classy version of this.