As a mom of a daughter who has had GERD since birth & who is now 14 years old, it sounds as if that may be the case with your daughter. It sounds as if most of her difficulty happens at night, am I correct??
And GERD sufferers usually do experience most of their problems at night. Since she has already expressed that she feels relief while sitting up I would suggest you look into the bed elevators that are out there to elevate the head of a bed. That could be a start. Also limiting foods 5 hours before bedtime would be best. That way most of the contents of her stomach have already left her stomach. Avoiding foods that worsten GERD would be smart as well. Spicy foods, tomatoes, citrusy fruits and veggies like onions and peppers as well as caffeine, chocolate, carbonated beverages are some of the culprits. Giving her a dose of mylanta before bedtime would help as well....if that doesn't work then one 10mg Pepcid should do it.
If this persists...it might be wise to have her evaluated by a Pediatric gastroenterologist since ongoing GERD could cause other probelms such as espohagitis and if left untreated could lead to stomach and espophogeal cancer.
I hope this helps and wish the best to you & your daughter.
(((HUGS))) ~ Goody