For something healthy, whole foods, and quick that can be bought at the store and kept around if something fast is needed: Larabars. They come in a good variety of flavors, are all natural, whole, raw foods - not processed/cooked - and are made of things like dates, walnuts, dried fruits, coconut. Calorie dense, small, easy to eat, and (of course individual tastes vary!) and I really think they're tasty (but I love dates, almonds, walnuts, coconut, etc.)! More info about them at: https://www.larabar.com
From the website, each bar gives you:
1 1/2 SERVINGS OF FRUIT * NO ADDED SUGAR * UNPROCESSED
RAW * NON- GMO * GLUTEN FREE * DAIRY FREE * SOY FREE
VEGAN * KOSHER
I would keep these around when I was in my 1st/2nd trimester, losing weight from nausea/morning sickness, and would find myself getting ravenously hungry all of a sudden. Of course, CF is a very different, much more serious situation, so I don't know if that recommendation applies her, but thought I'd throw it out. :)
Hope you and your daughter get some really great suggestions and that she's able to put on some weight soon!