She wouldn't have to cut the batting and the backing into the same width as the Jelly Roll Strips. Cut the backing and the batting in lengths she could comfortably handle on her sewing machine. Sew the strips on until she is next to the last strip, then cut another baking and batting the same length as the first one and continue sewing the Jelly Roll strips on. When she has it as wide as she wants it, the only thing left to do is bind it. First stitch all the way around the top, then sew binding on.
I would suggest 14 inch width for the batting and backing (make 6) for an 84 by 90" quilt with the strips lengthwise. For 84 by 90" quilt with crossways strips, but batting and backing into 15 inch widths (make 6). I would suggest allowing half an inch for seams.
I have made crib quilts in this manner. It works, is fast and very neat. If you use a fluffy batting, it is beautiful.