Everyone makes mistakes. She was trying to help you save on fabric. I would go back and just joke with her about the mistake and buy the extra. If she had measured correctly the first time you would have had to buy more anyway. Piecing the backing is no big deal.
If you don't want to go back you could always split the backing down the middle and add a strip of a blending fabric. I have seen people do this and it gives the back a little pizzaz.