It could have just been a huge huge coincidence. Maybe if another pretty woman had been in the bathroom then she would have said the same thing and been completely wrong. Maybe she had worked up so much anxiety about running into this new girlfriend that she was going to expect every woman she crossed paths with to be her. It also seems obvious from the story that the new girlfriend goes to the same school so it isn't THAT shocking to have run into her where she did. It would have been really crazy if it was at a McDonalds on the other side of town or something.