My best friend keeps asking me to be roommates and I have already told her I wasn’t ready. How can I get her to understand?

A couple months ago I told her it’s better if I waited until I finish college all together and then I’ll see. She understood but yesterday she told me her now ex boyfriend and his family are kicking her out, she is looking for a place and asked me to be her roommate. I was worrying about doing my homework. I had to focus on that. I can’t afford to move out right now and plus I help my dad out with the bills. As a teacher’s assistant, I don’t make much with the $14.50 I make and the school year only goes to 9 months but I’ve only worked there for five months. Anyways she said I am the ONLY one that she trusts and I won’t hurt her so she only prefers me to be her roommate over anybody else. I warned her about moving in with her ex boyfriend before she moved back. She lived with them for a month and she is moving out soon. I can’t move out right now. With the estimate $17,000 (not telling you how much a really have) I have all together (savings and checking combination), I can’t afford it. She needs to find a job first and I know she is focusing on her classes in college.