Since this does happen, (even if we don't know that this guy did anything to this girl.) then I think a bigger question to ask is pretty simple and will surely do better in the long run. When someone around you acts like a pig, girl or guy (I've seen some girls act downright crazy when alcohol gets into the mix, it's the thing to do when you want to act out your deepest desires, liquid courage I call it.) call them out at it. If you see someone groping someone else when they don't want, let them know it's not cool and you won't stand here while they do it. Same thing for any sexual advance that is unwelcome. You can usually tell when it's unwelcome when the receiving party looks nervous and tries to politely get out of the situation.

The below kinda-quote (it's disputed if it was Burke, or if it was said exactly thus.) comes to mind and rings pretty loud.

"All that is necessary for the triumph of evil is that good men do nothing" - Edmund Burke

As an exiting thought, the pressure to be civil needs to come from peers. Meaning, guys don't let guy friends harass, and girls don't let girl friends harass.