Sure, I perfectly understand how bars work. Like you say, it's totally possible nobody at the bar knew this woman was being harassed. Especially if this guy has plenty of experience being a creep and has gotten good at doing without attracting attention.

What I'm saying, though, is that the people around them at the bar are far more likely to be able to assist than a conference organizer who's not physically there.

"The point is that conferences should have a policy for how to deal with these kinds of situations. Those policies are probably going to depend on a certain amount of judgement from the conference organizer."

I totally agree that there need to be policies. But policies are just words, and they won't get the job done alone.

What will get the job done is looking out for each other. In social situations, particularly bars, we should all be looking out for each other, especially women, since they're disproportionally the ones on the receiving end of predatory/harassing behavior.

If she went to the bar with a group, every single one of them ought to have been looking out for each other. And yes, I've been in scores of similar situations where I and others have looked out for the welfare of others. You don't need to be an obnoxious "white knight" about it; you don't even need to be overt. Creeps tend to look for girls whose friends aren't paying attention.

Example: Your female friend is caught in a conversation with a potential El Creepo. Play dumb and introduce yourself to the guy in a purely friendly way. Heck, maybe buy a round of drinks. Just knowing that somebody noticed him will often nip things in the bud.