Be sure to tell her that t-shirt quilts are quite heavy. I made over 6 t-shirt quilts and I used the lightweight Pellon with no problems. I used a 15" square to cut the shirts after you apply the fusible interfacing. I hope that you are not planning to hand quilt this quilt.
Here are some tutorial videos to watch. Good luck Don!
https://www.google.com/search?q=yout...utf-8&oe=utf-8