If this happened to me and I was in your position, I probably would end up taking a "delayed action" response. What I mean is that I would sit on this, not think about it, and move on with my life for the time being. Beyond that, a steady chance of finding opportunities may present themself to you where you can end up getting him back later in a possibly much worse way. You don't have to smack him back, but you can opt to not support him in situations that he would need it in. AKA, let him get his butt kicked by someone else and not bother backing him up.
But... to be honest? What I just said sounds really spiteful, and only someone with my vendetta mentality would accept the kinds of sick and twisted revenge scenarios that I would wish upon my foes. I hope you aren't that type of person.
Just... ignore this post, please.