Could be a number of things, but I'd guess that she's experiencing acid reflux. I had the same problem for a number of months a couple of years ago or so. Acid reflux is associated in people's minds with heartburn but very very often it is not experienced as heartburn but as nausea or a vague feeling of unease or slight queasiness after eating. It can be quite uncomfortable. That's what I was experiencing a whole lot. I talked to the pharmacist at my local drug store and he unhesitatingly advocated buying OTC Prilosec, which is a brand name for omeprazole. There are other similar drugs OTC, and you see them advertised a lot on TV. The procedure for taking them is to take them for a month or so, IIRC, and then stop. I found this moderately effective.
When I saw my doctor, he prescribed omeprazole (which is the same thing). I found a cheap source online, a lot cheaper than getting it through my HMO's pharmacy. I still have some in the freezer, but I don't need it these days. I grew out of it. In my opinion, it was caused by anxiety.
Exercise is something I always do. However, I'd advocate that your girlfriend incorporate exercise into her lifestyle, because it will tend to alleviate anxiety, reduce stress, set in motion natural physical processes that will tend to alleviate the problem, assuming that it's what I think it is. Also, she should try to eliminate stress in whatever ways is necessary. HTH.