Some migraines are caused by food allergies/sensitivities. I get mine from processed cheeses, red wine, chocolate and caffeine. Migraines are caused by the blood vessels in the brain dilating and then constricting rapidly. Stress may also play a role in having one as well as hormones. A lot of women get them just before or at the beginning of their menstrual cycle. Hope she feels better soon.
It's unarguable that Americans' fatty diet is responsible for an excess of health problems in the US but it would be uncommon for such young women to have heart problems already from their diets. The most common health issue from dietary fat is obesity, and from that, diabetes. Nobody is sure about migraines except that they have a genetic link and chemical or hormonal triggers or both, and we do have a lot of chemicals in the processing of our foods, so that could be part of why there are a lot of migraine sufferers in the U.S. It could also be that we diagnose migraines more here and that there is the potential for there to be as many anywhere. It is widely understood that people who come to this country from places with better diets and who change their diet to eat the way Americans do will get less healthy. A classic group to prove this is Japanese immigrants who begin eating the McDonalds food and get strokes and heart issues in one generation.
I would advise your girlfriend only if she asked me, but looking at her diet is not a bad place to start. If she has had heart problems, she probably has had diet advice from her doctor already. They may be American doctors, but cardiologists aren't stupid about diet.