I'd go back on a day when she was there, explain and ask her what you should do. If she wants you as a happy returning customer, she'll make it right. Word of mouth is an amazing thing.
If she doesn't make it right, then purchase (ELSEWHERE) the material you need to give the back a border. I've seen backs before that had a strip of pieced materials from the front that looked neat. I'll see if I can link that.
http://www.quiltingboard.com/t-150277-1.htm#4121560