Can you make one like a smock that you put on like an oversized vest and have the tie straps on the sides or near the front? She could just put it on and overlap the two sides and tie it closed.
Another idea would be to make one with elastic at the waist so she could just slip it on either over her head or step into and pull it up.
Yet another idea would be to make one like my second suggestion but, instead of elastic, put a drawstring in the waist that comes out of holes in the front.
Either of the last two could be made using a traditional pattern, but, instead of having the tie straps on the sides, make a tube to go around the back for the elastic or the drawstring.