Wow! That is pretty unbelievable. I LA and if someone gave me a backing that was not the right size for the top I would contact them and tell them the backing was too small.
Now, if someone gave me two sheets and expected me to piece them to the appropriate size backing and then cut down I would do it but there would be an additional charge for that. I expect backings to come to me already peiced and cut to the appropriate size for the top. Backings need to be 6" bigger on all 4 sides so there is enough to attach to the leaders, account for a top being out of square and to test tension. But to cut down a top to fit the backing?? Not good, she should have contacted you prior to cutting anything down.