If she is able to see well, I would get her a nice worsted weight yarn and size 8 or 9 needles.(Lion brand Homespun makes really nice scarves too) If she would like to use chunky yarn to use, you could go with a 11 or 13 needle, they go really fast on these, and turn out really nice.
Check out the nice purple, tan , white and green scarf here on this forum that May has posted a picture of. It is a beauty. It would be easy as well, as she could use any stitch that she knows how to do to make an interesting scarf.
Happy knitting to your Auntie!