She really needs to have this checked. One possible cause is Celiac disease, which would cause gluten to make her sick. Gluten is in so many foods it is hard to figure out on your own. She should get tested for gluten allergy before stopping gluten. She can feel much better.
There are many medical things that can cause someone to get sick after eating. Anything from allergies and intolerance to hormone fluctuation and stomach ulcers. She really needs to see a doctor asap.