Hmm... seeing has how you both must be about the same size (she's wearing your shirts), "borrow" some of her clothes without asking. She may not like it, but fact is, she set the ground rules for your friendship. The problem with dealing with people who "don't play by the rules" when we do is that they have no sense of guilt for their actions (stealing, lying). Assuming you aren't going to press charges for her stealing your stuff, the only way justice may be truly served ("eye for an eye" theory) is to return the favor. Quid pro quo. Tit for tat (no pun intended).
Her actions are apparently "OK" to her. Therefore, how can she object to you returning the favor? Now maybe my suggestion may offend the "God fearing" amonst us, but if there is any chance of your friend learning any lesson here, you need to dish it back. Friendships that last are based on a relationship of equality. You do them a favor, you expect them to return the favor when you need it. You let them borrow money (and I NEVER advise this!), you expect them to pay you back, as well as let you borrow money when you need it. And you pay it back. They borrow your shirt without asking, return the favor.
Just my opinion.
P.C.