I agree that the LAer should have called you but... I would never have accepted a sheet for a backing and two sheets would have been way too much. In other words, you expected her to take two sheets, cut off the hemming, sew the two together, cut it down to size, launder to remove all that stinky sizing and then press the whole mess. While a sheet seems like a good backing option because it is one big piece of fabric, it is not appropriate fabric for quilting. The thread count is usually too high and makes it difficult to quilt through. It was unforgivable for her to cut your top but also unforgivable for you to expect her to make your back for you. Bad situation, no good solutions. I think she needs a new customer and you need a new LAer.