A good old chicken soup with pasta it in?
I'd also say any casserole type of thing served with mash pots and nice fresh veg.
I'd probably also do some old fashioned milk puddings if she likes them.
Custard and stewed fruit, rice pudding, bread and butter pud etc.
Her weight will soon even back up - much of what it lost is fluids, so do concentrate on getting her to drink plenty
Hope she's back to normal soon