it would be more usefull to her if you got some second hand bricks and built it about two foot six , put rubble in the bottom then soil and finish off with the compost , she will be able to garden at a confertable hight, even by siting on a kitchen chair, and leve the choise of what she grows to her , she was not born yesterday, and you can check every thing with the people on hear,a number of us are around her age and some older.by the way she will need a path all around it ,so that she can reach everything,

by the way welcom to GOY you are very thoughtfull, good luck with the prodject,