No. Not reacting physically. I would probably not say anything at that point. But as the day/evening/night progressed I would just keep a quiet eye out and see if it kept happening with her or others but I would leave it at that if nothing further happened. If he kept making advances then I might politely let him know that we are together if the opportunity arose.
But otherwise I'm not too fussed. It wouldn't really effect my night.
In realistic terms and you weren't to know that, but DP doesn't like crowds and is not fond of being touched much. So we wouldn't have been there when it got busy. Also, if she was uncomfortable being touched like that by a strange person she would tell them so.
But I understand your scenario.
Your second scenario gets under my skin. Especially if I imagine it being another man that's she's exposed for. Regardless of the circumstance. My heart rate has actually increased just telling it to you.