I too made quilts from my FIL's shirts when he passed. I made one for his wife, my husband and my SIL. They were all very pleased with them. I used his shirts, and purchased a coordinating fabric to go with them and just made a 3 part rail fence block. Some of his shirts had his name on the pockets, I made sure to have one in each lap sized quilt. I then made a label that said in Memory of...... Keep it a simple design and it will come out nice and be very much appreciated. Be sure to fuse the back of the t-shirts and work them in too.
Good luck, and you and she are in my prayers.