oh my...that is so sad...i always insist on an extra 5" all the way around for backing & batting so i can avoid that situation...if a customer's quilt comes in with the backing too small i let her know & send it back to be 'fixed' I actually have a quilt with the exact problem...it's one of my own & i don't know what i was thinking....i stopped quilting it near the bottom when i realized my error, unloaded it & stuck it in my closet...now i use it to show people when they wonder why i always insist on extra...maybe someday i will figure out a fix & finish it- but for now it's just a sad example. it's too bad your's is a customer one & not one of your own. I hope she has the extra you need to finish it up- it sounds as if the other option (cutting the sides) may be a workable solution too...take a deep breath, many people have done this- you are not alone...but do learn from your mistake & always insist on the extra...and measure, make sure when the quilt comes in so you can give it back if it's not big enough....take pictures of that one so you have an example to show others.