Co-Signer Advice

My best friend and I go to college in NYC and want to move into an apartment for our senior year, but she does not have anyone that could co-sign for her. My mom can co-sign for me but if my friend does not have one are we unable to both be on the lease? Would I have to sublease to her, and if I do would my mom be solely responsible if either of us misses rent? Please any advice would help so much, thanks!