My…”Friend”

My…”Friend” is bulimic and throws up alot.
Sometimes it’s just when she eats a lot of fatty, greasy, foods but lately it’s been everything.
I don’t know what to do.
If I tell, then she’ll be pissed.
If I don’t… i don’t want ot know.
She’s thin as a rail and athletic to.
What should I do?