"How" or "why" is she a phony? Like, does she pretend to be lil' miss priss and good girl around her parents, the roomie's parents, a TA in class or a "dorm/house mother" or any other form of authority...so that the outside party thinks she is a great student and a "good girl"...even though she's a wanton slut? Or what? lol
emdless possibilities, depends what you like. Your suggestions are good, or there's- she pretends to be prim, proper, but when Gal A brings guys back to the room to makeout or have sex, she's noticed Gal B only pretends to sleep, but is secretly watching and masturbating...
A) Take nude pictures of her, perhaps her masturbating, or blowing a dude. And then blackmail her with it so she has to give them sexual favors so they won't destroy her reputation.
B) Gal A could have her guy friend seduce her to prove she's a slut. Gal A watches them fuck, and becomes aroused. It eventually forms into a threeway relationship.
(Disclaimer: but of course these things are terrible in real life...just terrible..except for in a story)