Does she have enough mobility to bend down and attach elastic? When leaning too far forward in a wheelchair, it's very easy to fall out. Ask me how I know.
If she is totally not walking, then I would do a foot warmer type thing. Make the quilt rather narrow, make a padded piece as wide as the quilt about a foot deep. Sew it to the quilt with wrong sides together and round the corners. Do not sew across the top edge, of course. Then flip it. Viola, a pocket for the feet.