There's some good advice here! I would add that I think it's important to let her know that you will no longer quilt for free before she starts a quilt that she expects you to quilt for her. Don't wait until she has one that you have to refuse. You could suggest that she explore QAYG techniques since she seems to be able to quilt smaller quilts herself. I've started to do that and love it!
Good luck. You were so generous to do the first (and second) ones for her. She should be counting her blessings.