Honestly when he grabbed her ass your buddy should have walked away. If she was still interested she would have chased him. If not he's already talking to the next girl.
High value applies to your time as well. If she deviates from your attention--even if only for a matter of seconds--walk away. Unless you're exclusive, men don't wait on women. It's her way of giving a shit test. How much of a p*ssy is this guy? How long can I make him wait before he does something? You aren't her b*tch. Walk away.