Is she paying rent? If she pays the rent and you accept it then you have a landlord-tenant relationship. If she has not paid any rent, then you may have to file an unlawful detainer action. Either way, you are probably better off retaining an attorney for this problem.
I provide a free 15 minute telephone consult for security deposit claims and eviction defense. No attorney-client relationship is created by answering questions in this public forum. If you wish to create an attorney-client relationship, you must contact me directly and sign a representation agreement. Answers are provided based on general ideas and an answer specific to your situation would require a review of all documents.