... I would argue that she clearly has a definite disability going. Mental illness - which bulimia/anorexia (in all likelihood) is - is an invisible, but real, disability itself.

And "normal" stalls are cramped and uncomfortable and often a lot less clean. Seriously, you recognise that she's clearly not fine, but that's as far as your capacity for empathy goes? "Well, sure, she has serious mental health problems that could very well kill her, but she's using the handicapped stall, and therefore she just flat out sucks as a human being."

Tell her you need to use the stall, give her a minute to clean up, and perhaps try to recognise that invisible disabilities are still disabilities and can be utterly crippling.