I have a roommate that isn't paying rent?

My girlfriend and I have been sharing an apartment with a friend for 2 years. The first year of our lease we had a room mate we had no problems when he moved out we were going to look for something smaller. Another friend said that she would like to live with us and immediately signed a lease. Because of a problem with my co-signer I'm not on the lease. My other 3 roommates are all on the lease. The girl that signed the lease never moved in because she broke up with her boyfriend in the area. She paid rent for the first four or five months but now refuses. We let it go two months because I don't want to get kicked out because I'm not on the lease. All 3 of us living here are very financially strained and need an answer. I don't want to trash my friends or girlfriends credit by getting evicted so we feel like we are in a pickle. Any help would be appreciated.