I am in the process of making my husband's quilt bigger. I had to rip off the binding on two sides, and then add the new blocks, batting and backing by Marti Michell's encased borders method. It's a big pain, esp. when the quilt is ending up so much bigger. Now I need to quilt it and re-bind it. It's a lot of trouble. Would have been easier just to make the top bigger before quilting and finishing the quilt.
I would just say no, you can't do it, and don't offer any further explanations. If you don't think it's too cheeky, you might tell your boss that you make quilts for sale... $XXX for the size she wants.