Hmmm. I'm wondering if this was just an excuse because the quilt wouldn't fit her frame without cutting it down a bit.
I'm assuming that she returned the fabric that she cut... You can add a border to a finished quilt, as long as you haven't bound it yet. You can use a qaug method. There was a posting on this very topic recently. I'll see if I can find it.
ETA:
Ok Here it is:
http://www.quiltingboard.com/t-137222-1.htm