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Look at it this way, if your in a pub and a mate gets into a fight your going to jump in aint you? Even if you know your mate capable of sorting it himself?
This is a silly answer if it's one on one why would you jump in unless it was to stop it. Too many gutless wonders out there jump in and gang up on people nowadays. If he is stupid enough to get in a fight and can handle himself let him wear his own lumps. Don't escalate it that's where things go pear shaped and someone ends up seriously injured.