Glad her GP is investigating. Weight gain requires as much planning and application as weight-loss. If her appetite is jaded she has to choose tasty, calorie and nutrition-dense foods in small amounts but eaten very reqularly.... literally something every hour or two whether she feels like it or not. Fats are ideal... so things like nuts, seeds, avocados, cream cheese, peanut butter, tapenade, banana or nutella on crackers/biscuits. Protein-rich foods combined with light exercise could help restore lost muscle... eggs, meat, fish, beans, nuts, tofu etc. Fruit juices and milky drinks as suggested early
Things to avoid would be the opposite of the above i.e. bulky, low-calorie or low-nutrition foods... so not filling up on too many high-fibre foods, vegetables, salads or water.
And, until the doctors can work out what's wrong, a good quality multivitamin with iron wouldn't go amiss. Hope she's OK.