It is similar with late-stage cancer sufferers. Sweet, salty and soggy seems to work. You don't have to spend hours cooking, though the idea suggeste by FifiMo are good ones, i.e. half supermarket and half home added ingredients to get the nutrition levels up.
Trifle is a good one - tinned or fresh fruit in the jelly, plus a jam swiss roll, custard with full fat milk, and double cream on top.
Scrambled eggs with finely chopped ham, cheese and chopped tomatoes, just on its own, no toast unless mum likes toast.
Bread, cheese and milk soaked and heated, with maybe added cream. A pinch of cinnamon or a clove might add flavour.
Soups - all kinds - with lentils added for protein and flavour, you can't find them once they are absorbed into the soup - and grated cheese on top.
If you don't want to cook from scratch, cheese is a tasty addition to a lot of canned soups or mixed with sardines. Tuna and mayonnaise.
Maybe three small courses will work - soup, tuna and mayonnaise, trifel. Only a little of each.
Hope it helps.
Love
Margaret