I think even in that situation the best thing is for the person who it's directed at to say "hey, that ain't right." Simply because that's the most powerful way to deal with it. Anyone else is going to be treated like an outsider by the "attacker". I see it happening all the time on different forums. Other people getting involved inevitably means the argument growing and discussions getting derailed. And in the end pretty much no one changes their behaviour.
We'll have to agree to disagree. I believe that public disapproval of antisocial behavior is the most effective way to stop it. In bullying situations, if the bully's victim is the only one who ever says something, everyone believes that the victim is standing alone -- because, well, it's the truth. It's when others publicly express their disapproval of the antisocial behavior and their support for its victim, in a way that the bully, the victim, and everyone else sees it, that things change. The famous words of Martin Neimoller come to mind here.