If it's a very small incident, why is it such a big deal for her? Especially compared to the real harrassment by her mentor in the car?
> Over time this began to really bother me simply because of how small and benign it should have been.
Well, did you ask your coworker why she did that? Being an adult also means standing up for yourself (it doesn't have to be overly confrontational, just ask politely), especially since it apparently was just a single person, not bullying by your entire team.