In what sense? The simple one. It is nothing to do with him. Yes, he is a witness, yes he can leave his name with the lady concerned, but very simply THIS IS NOT HIS BUSINESS. With regard to the pimply yoof at STN, the correct thing to do is either call the police to report an offence, complain to the airline or report this member of staff to their employer or all three. There may be other options, but again, you are not the police!
If you decide to get involved with somebody else's fight make sure that 1. You are totally in the right. 2. You are prepared to miss your flight. 3. You don't make it worse for the person you are trying to help. 4. You win. 5. Make sure the person really does wants your help.
Doing nothing when witnessing things like this is not an option, but make sure you do the right thing.
PM