I would find out how many feet you could sew in an hour, then charge by the foot. Charge only what your friend and you feel comfortable with. Make sure that both of you understand. Usually because of my set up I could easily square a quilt up in 15 minutes. Be sure to get an approximation on how much thread is used. Sometimes customers will supply their own. Take it one step at a time. This could bring you in some extra cash which most people these days could sure use a little more.