The title of this post makes me physically ill!! I would JUST DIE if this happened with one of my customers.
Here's how the stretching can happen. Let say you have 1 piece 35" and the other 35.5" long. If the longer piece is on the bottom, your feed dogs can ease in that extra 1/2". This technique is done (on purpose) in the garment industry for things like setting in sleeves. I've had borders come to me several inches too long because of that "feeding" or stretching issue. This also can happen on backs, especially if you are seaming a back and don't have both pieces exactly the same size. You won't be able to see the easing in the seam, but it shows up in unexpected places. I cringe when I see pieced backs (lots of pieces), just because there is so much room for something to go wrong. Add in mystery fabrics, a little bias and you have a wreck looking for a place to happen. A little fullness on the top can be controlled and hidden. A little fullness on the back is not always easy to spot until it's too late.
Now..for your current issue. It looks like from the pictures you might be able to do a slip stitch to tack down those pleats and no one will be the wiser. Your back will be fine. Talk to your quilter, see if she offers a discount or refund. I know that if this were me, I'd want to make you happy.