Roomie's MOM stays over nightly...?

SOO... I am a senior in college and have a roommate whose mother stays the night frequently. Her mother moved here from California last April and at first I didn't really mind as she was getting settled in. But this has become way to frequent. Out of the past 21 days I can honestly say that she has at the very least spent 10 of those nights sleeping over at our apartment. To me, this seems unfair because if I want to have guests over or have a small get together I am unable to do so on behalf of the fact that her mother is nearly 60 years old and stays the night!! For example this past weekend was our schools homecoming and I had told my roommate prior to Saturday night thatI was having people over to drink before going out and our drinking was cut short due to her mother being present. I also do not see how it is fair to share a 2 person lease with my rommate when her mother is her about 20 of the 30 nights of the month regardless of if she has a car or not...or whatever..HELP!!