Saying this is a murky situation, saying some girls like it (I don't care if you qualified it as "some" girls)... not ok. The only time it's ok is if you know the woman and know she's ok with it. It's NEVER ok when it's a stranger.
Yes, someone else should have spoken up about it and said "Hey, dude, WTF are you doing? Leave her alone." The fact that no one did, when it was probably pretty obvious she didn't know the guy, is part of the problem.
Yes, she should try to make it clear she's not interested, but it also shouldn't be her responsibility to make sure she's not sexually harassed. Just like it's not my responsibility to make sure I'm not mugged. A tech meetup is not a place where you should have to worry about that sort of thing.