To the younger folks out there: if you see something like this occur as a third party (you're neither the groper nor the gropee, neither the misogynist nor the victim), you need to not politely ignore it but to step in and correct the asshole, as firmly as necessary. Whether it's groping or just verbal putdowns of some sort, you should step forward and add yourself to the situation, making it clear that the behavior…
As to stepping in and "adding yourself to the situation" with a stranger? I don't know about that, so much. I've had my fair share of trying to be the considerate stranger stepping in to help out, only to be screeched at by the person I was speaking up for. It often doesn't pay to try and help people out. (Not saying you shouldn't do it, anyway, just on principle).