Well don't you think that is kind of a backwards way to work it? I wasn't raised to let people walk all over me, but you have to pick and choose your battles. In this case, beating the guy's ass would only result in bad for you. Aren't you in college or work at one or something like that, would your employer like that you were charged with assault? What about future employers? Think about what you could lose over this.Dunno man. That's just not how I raised to handle things. If it happens again though, we will be pursuing legal action to protect my ass from getting into trouble he stirs.
Had he assaulted you or her first it would be a different story, but with the info you gave us I can't justify you smashing his face in and being justified in doing so.
Never raise you hands in anger, unless you're absolutely sure you can get away with it.