My Aunt taught me how to make quilts when I was a teenager. She learned during the depression era. We took old clothes and cut out all the usable parts. We took old sheets and cut a stand size square from each one, I think we did 6" squares. Then we sewed the fabric pieces onto the squares, then flipped them over to trim even with the sheet square. Then we used army blankets that we bought from Goodwill and used them as batting. And we used either sheets or fabric that we bought on sale at TG&Y for the backing. We would pull the frame down from the ceiling and load the quilt on to it and the whole family would get involved in tying the quilt. My Aunt would used crochet thread for the tying. It was my job to hem the quilts on the treadle sewing machine.
Fast forward a few decades. I made my DH a t-shirt quilt. It was fun to make. It was a very simple design - 15 1/2 inch squares with 3 1/2 inch flannel sashing with a 6 inch flannel border and flannel backing. Warm and natural batting. It's a nice, warm snuggly quilt. Make sure t-shirts are washed without fabric softner and don't use fabric sheets. You would want to do this yourself just to make sure. Also, Pellon interfacing on back of t-shirt area before you cut to size. I tried using nylon invisible thread but it kept breaking on the t-shirt fabric. Someone on this board told me about using Sulky polester invisible thread and it didn't break. T-shirt quilts make great memory quilts.
Made both my boys jean quilts from their old jeans. One quilt I did 9 1/2 inch squares which also included some pockets and some seams just for fun. Makesure seams are not near the corners. I found bandana material at the store and used red bandana material for the sashing and blue for the cornerstones. For the other quilt, I used 4 1/2 inch squares alternating with blocks made up of two rectangles squared at 4 1/2 inches. Also used a flannel backing on his. The boyes loved their quilts.
Just use a basic design. Sounds like the fabric will mean more to her than the design. Just keep it simple so you don't get frustrated.
Good luck and let us know what you decide.