That's sad. I had one of the kids that stayed here take my credit card. He kept it for almost a month, after he moved out! He only racked up like $300. But, he kept telling me he didn't have it! He knew all he had to do was ask and I'd give him money for gas.
Anyway.... to get my card back, we had to threaten to call the sheriff. He told me he found it in his truck. LOL
He's no longer allowed here. Which is a shame, this is one of the kids who stayed here from his freshman year in high school until he was about 22. But, he must not have cared too much if he was able to steal from us. He still calls every once in a while. And when we're out somewhere, he'll talk to us as though nothing ever happened.
Ask her to leave. She isn't your responsibility and if she isn't ready to help herself, she's only going to keep hurting you and roommate A.