Katie, a casual friend

You recognize the voice and reply, "Hey, Katie." You move to a stall, thinking it's free. Opening the door quickly, you jump back with a start as you stare at Katie. She's lying there, her skirt up, and her fingers inside of her panties. Her eyes spring open, and she tries to pull her skirt back down. "Uh... John ...I..."

You blink a few times and try to decide what to do.