I have experience with controlling weight loss. But I am not a fundo patient.
Usually I add food of high calorie value to my diet. Seeds and nuts of all kinds are my favorite. Almonds, walnuts, cashews, peanuts, pistachios. They offer 500~600 kcal/100 g. A GERD patient may not tolerate them. You need to boil them till they are soft to chew. Start with small quantity to have the body gradually build up tolerance towards them.
Beside the nuts, you could also try Ensure Life formula. It's 400kcal / 100g. You can take four scoops in the morning snack and four scoops in the afternoon snack.
Honey is also another candidate, quite tolerable to GERD patients. It's only 300kcal/100g. So you need to add more. But they taste good.
Good luck. I wish your daughter will get well.