What can I do if the landlord wants to evict me if I don't get rid of my roommate?

Well, unless you have a lease agreement with your landlord, the proper party to “evict” your roommate would be the landlord. I would question whether or not you have the “authority” for force your room mate to leave. Obviously, I would need to read all relevant agreements to see who has such authority to “request departure”.

If your roommate is not on the rental agreement and will not willingly leave, you can have her evicted by giving her a 30 day notice to quit and a 3 day notice to quit the nuisance. Then you will have to file an unlawful detainer complaint if she hasn't left. After getting a judgment against her for possession you can get what is called a writ of possession and the sheriff will evict her. Keep your landlord informed of what you are doing so he doesn't attempt to evict you in the meantime.