A very good friend of my dad's died in the street in a similar situation. I never met him but from what I gather he would not hurt a fly, but some scumbag waited for things to cool off after a few words were exchanged and sucker punched him on his blindside just like this. He was dead before he hit the ground. The guy he hit him came from a good family but is now serving time for manslaughter.
Since that situation, I cannot view something like this as ok or deserved. If, as it seems, he had decided he was going to step in, fine. Push him, square up to him, let him know you have a problem and you want to sort it out. Then fight him if you really must. The guy probably deserved a lesson, but punching anyone blindside to the temple is being a scumbag IMHO