Welcome to health boards, Vzent . . . it is very possible that your girl friend has food and even additive sensitivities. I have IBS (irritable bowel syndrome) and have through years of tracking what I eat and my symptoms discovered things that I should never eat.
I have to avoid fruits and fruit juices (except bananas) because of acids, additives in lunch meat products and a lot of frozen foods, MSG (monosodium glutamate) and anything fast food that is fried.
I was in my late twenties when I developed this condition and I believe it was brought on by stress.
The endoscopy will be a great thing to rule out anything serious. If she is having trouble after eating certain foods, she needs to quit eating them and see if she feels better. After a while, you get to where you no longer even desire to eat the things that cause you trouble.
Let us know how you two do!