It's unfortunate, as @Cynthia has said, it's a common exploit, one discussed on the forums here many times. Luckily for all of us, there is a new feature according to the new cancellation policy, that we, as hosts, will be able to see how often a guest has cancelled before we accept them as guests.
Go to her profile and flag it for good measure and flag the conversations in your email where she requests to change. I think the more we point out directly these types of issue, the more likely that Airbnb will do something to change it. Also, you CAN go to the Resolution Center and request money from her, I believe. She'll probably say no, but you can make sure Airbnb knows about her little scam.