The only thing I can figure is that the quilt shop heard "36 x 77" instead of "66x77" for the size of the top.
That's the only possible way that I can see that the yardage she purchased would be enough.
Human error. We've all done something similar.
And if she's upset at the cost, it's better that she starts to understand now that quilting CAN be an expensive hobby.
Of course, any hobby can really get expensive... my son did motocross racing, we've done horse showing, my daughter was in band... so, in comparison, quilting can be considered much less expensive than any of those.
Just a matter of perspective.