Not directed at me, but this is a great question. If I saw a neo-Nazi getting sucker punched, I wouldn't physically intervene unless it was clear there were some way I could do so without escalating things. I wouldn't chase down the puncher, but if the NN needed medical attention after I'd call 911. If I had seen or photographed the puncher (recognized by me or not) I'd call law enforcement and give whatever evidence I could in the case.
If I suspected I knew who the puncher was due to something admissible in court I'd turn that over. If I knew for sure who it was I'd try to get him to turn himself in, then do so myself if he hadn't after a reasonable time. If there were some extenuating circumstances (e.g. he got beaten up a lot as a kid for being the local 'Jew boy' or whatever) I'd sure as hell get those facts known as well.
To me the minimum expectation for a citizen in such a situation would be they'd lend assistance in whatever way they could without endangering themselves or causing some undue burden. That's what do do out of respect for the rule of law when anyone is victimised. Beyond that, when the victim is more sympathetic, I'd personally go above and beyond that, to varying degrees depending.
Basically, there's a certain bar you should be expected to meet for any citizen (or resident, or whatever) who is the victim of a crime. When the victim doesn't have a reprehensible outlook you go much, much further than that.
Edit to add : not part of the question, but I'd also note that anything / everything the NN may want to do for or give me by way of appreciation for my assistance would be flatly refused. I wouldn't have done any of it for his individual sake.