This is an awful amount of disrespect.
I couldnt sleep at night knowing that I innocently got slapped and that the dude could be found.....
If it were a random punch up and it kinda got put down to a boozy night, then fair play, but they being connected to you like that.....I would get in touch with your bud, and tell him you want the guys number. Call the guy, and tell him that he's a **** weasel and that you will NEVER forget this, and you will be actively seeking revenge in the way of giving his mother, sister and new girlfriend anul sexx ASAP.... fook him bud - get your fookin revenge!!!!!