ANY form of intervention would be better than none.
In that scenario, I'd walk over and say something like:
"Excuse me, hope you don't mind me approaching you but I'm just a little perturbed at what I just saw. Do you mind if I ask if everything is alright?"
I'd then ask the lady if she was comfortable with the situation and invite her to leave if she so wished, without fear of intervention from anyone else.
If matters escalate form there so be it, but I certainly wouldn't stand idly by and watch. If it's horseplay then I'm sure both parties would understand my concerns.