I was hoping the kids' response when asked to downgrade would have been to call the FA and ask to have him/her pass the message along to his parents in J that the nice man sitting next to them asked them to move back to Row XX, so his family could sit with him. That would have nipped it right in the bud!

I get the sense here that the OP's friend's issue is more about accountability for the galling behavior. People like like that eventually get theirs. It's the law of karma.