I agree with b. zang about machine stitching the binding front and back instead of hand stitching the back. An I Spy quilt is usually for a child and the machine binding will hold up a lot better.
As far as doing it as a favour--that can rebound in two ways--the person might be offended, so a (small) fee should be charged. Or, the person might be so thrilled with the 'free' service that she might heap more unfinished quilts on your friend, so again, a (small) fee should be charged.