True, but the trouble is, I suspect, that having reported it (does she have time to do it there and then, or will it get left to the end of a shift), what's the comeback on the perpertrator? He's probably long gone, anonymous, and thinking he's got away with it. It's not quite the same as an action by a colleague seen on a regular basis.
I think I'd be tempted to whisper something extremely rude and slightly threatening to the perpertrator, just to let them know it wasn't acceptable, and then move on. If he wants to complain, he's got to explain what prompted it. (This is purely my idea of good revenge by the way, not advice!)