You should serve her with a 3 day notice to pay rent or quit, if she does not pay the amount within the 3 days file an Unlawful Detainer Action(Eviction) in court. Your 3 day notice to pay rent or quit needs to be perfect. Hopefully with the Unlawful Detainer action you can obtain a judgment for the back rent.
Since you are not the landlord, you cannot serve a 3-day notice nor institute an unlawful detainer action. Therefore, your only avenue is to sue her in small claims court based upon her oral agreement to pay rent (or written agreement if you have one).
For more information on how to file a small claims action, see the following website and its various links: