Wow. People can just make amazingly poor decisions. It is amazingly immature of your neighbor to spend time with this guy given what she knows or should know.
The guy is a hothead. And from separate reports, almost certainly abusive at times. But, he might be charming in other regards. Now, as far as helping your neighbor, there may not be a whole lot you can do. Often understating the case works best. Especially if she brings it up and you might say something in understated fashion like, ‘I’ve heard stories that he doesn’t always treat people right’ or similar, and pretty much just leave it at that. Getting in any kind of argument with her is probably a losing proposition.
Now, if he is engaging in stalking behavior toward you (and it’s always kind-of, sort-of stalking, hardly ever ‘textbook’ stalking) then any kind of interaction with you, whether positive or negative, counts as positive reinforcement toward him and his behavior.
It sounds like the Condo Association knows much of what has been going on, but has not really made an adequate response to the whole situation. It’s a shame they can’t send another guy, about the same size as this guy in order to reduce embarrassment, to act as your intermediary, and just matter-of-factly tell this guy that you simply wish to be left alone.