Agree on the need to see and know exactly what she wants.
The dresses may be lovely cottons .... or perhaps velvets, polyesters, slinky fabrics, etc.
How much wear is showing?
And remember ... this is not like taking yardage and cutting strips and squares. You will have to do a LOT of prep work to get the clothing into fabric that's ready to cut for the project.
As a simplistic and lower cost approach ... maybe she'd be content with a total scrappy look, like an adult I Spy!!