I hate running into any therapist's clients, and I let them know it too. I enter the waiting room 3mins before my session time so that they'll hopefully be gone by then. Once with L, I saw her client leave and I think I threw a little fit.

I wouldn't worry about the girl. But I would talk to your T about your experience and feelings. Your T can't tell you anything about the girl, but she should help you process what happened.