rule #1 for the future: set a firm price and get the agreement in writing before you accept the commission.
rule #2: when it comes to business, friends are not friends. they are providers and customers. ;-)
do you still have the quilt? if so, finish it and sell it. if she demands a percentage, give her $15. i believe that's the going price of "just a bleeping blanket" these days at walmart. ;-)
if you already gave it back, then consider all the time you have now for quilting and for new friends since you don't have to waste that time on her anymore. ;-)