She should sell by the lineal inch of the bolt. You say that bolts do vary in width, and this creates a simple solution. In your example, you gave a 17 x 42 piece. The 42 obviously came from a bolt that was >42 in, so there would be dross that if she did not sell it, she would lose money. Therefore, lets say the there is a bolt that is 60” wide and 20 yards (720 inches) long. If the entire bolt cost $72, that would be equal to $0.10 per lineal inch. If the subject piece that is 17” came out of this bolt, the charge would be $1.70 or 17 lineal inches x $0.10 / lineal inch. Doing anything to discount because the customer does not want the remainder on the bolt width means that she will end up losing money because SHE had to pay for it, but no customer will buy it.
Right - a lot of shops will charge you for 1/2 yard for a 17” x 42” piece. They might be nice and cut it to size for you though.