Most Helpful Girl
2 stories actually, although I wasn't the one being exposed :)
1. My friend is actually similar to "the girl [you] knew." Her undies were always exposed. I would subtly tell her and then she would cover up. But eventually she no longer cared, and flashed as she pleased.
2. Family beach wedding. My auntie, wearing a dress, was posing for pictures while my cousin takes her picture as I stood next to him. An unpleasant breeze had to reveal her spandex undies to the wedding crowd, though she was too busy modeling to notice..