the first thing I would have done is square up the backing. Then remeasure to be sure it's going to be big enough. Second, I wouldn't float the top. Then I'd baste it every 6-8 inches. Finally I'd charge for the extra time to do all this.
Next time, I might offer to help her quilt on my machine. Then she could learn the hard way why it's important to be accurate.
Or just refuse it, with instructions on how to fix it and then bring it back.