If she wasn't going to be up front with you when you first asked you shouldn't feel guilty for backing out. She put you in an unfair position and clearly gave zero thought as how you might react. Shame on her for doing so!
My advice to you is to tell her you're not going to co sign due to her lack of honesty in the situation.
Another issue is how is she going to pay you back when they are living paycheck to paycheck?
__________________
|