cell phone mishap

About a year and a half ago, I moved in with my cousin who was wanting to switch cell phone providers. I currently had a provider and we decided that I would add a line to my account, make her an authorized user and she would pay her line and I would pay mine. In between this time I ended up moving and changed area codes. I was informed by my cell phone provider that since I did this, the original contract was void to the best of my knowledge. I took that to mean I was no longer reliable for her line. She was still paying it at this time so I thought nothing about it. Today, I find out that my service is about to be interrupted because of non-payment and the account is over 700 dollars past due soon to be going to a collection agency. So, it is going to ruin my credit. I got in contact with my cousin and she told me that because I owe her fiance money (which has nothing to do with this and is a small amount anyway) she figured I could pay that bill and we would be even. I don't think that's fair for her to run up the bill and then hand it to me. she knows I can't afford that and she told me she can't pay it. I want to know what my rights are and what I need to do. I have an email stating that she "can't pay it" and doesn't know how "she got it that high".
What do I do? Can I get her to pay up?

You can sue her in small claims!