If you've checked out every thing you can think of and nothing seems to help, you may want to give exclusion diets a try. One food group would be dairy (not the lactose portion, the casein portion).
An, as screwy as it might sound, if she was tested via the blood test or the biopsy during endoscopy (EGD), a negative result could be a false negative. You might consider giving a gluten-free diet a try. Especially if there are any other small, seemingly insignificant issues occurring, like rashes, extreme fatigue, 'brain fog', variations in bowel habits, etc. The 'mesenteric adenitis' makes me wonder if that diagnosis could be tied into an autoimmune reaction such as is seen in celiac disease. Celiac can 'present' in some very strange ways.
Yes, she has been tested for celiac and it was negative.
Has she been tested for celiac disease? Is there any history of gluten intolerance in your extended family?