This could be due to many reasons; the most common is secondary to certain types of foods that can cause bloating and gas. The best although not simple way to figure out which meals cause this symptoms is by performing an elimination diet, by withdrawing certain meals, one at a time.
Keep in mind that some of the most common reasons to have these symptoms are lactose intolerance, therefore avoidance of dairy products must be tried. Also gluten intolerance (avoid food that contain gluten, which is found in wheat, rye, barley; common meals that contain it include pasta, bread, pastries, etc).
There is also a lot a hype among gastroenterologist regarding a proposed diet for these symptoms called the "FODMAP" diet. You can "google" the diet, and she should try to being on a "low FODMAP diet". It is pretty self-explanatory, but basically she needs to avoid foods that are "high" in FODMAPs, such as certain fruits, vegetables, dairy products, sugar substitutes, etc.
If this doesn't work, then she might need assistance from a gastroenterologist for further evaluation and testing.
Hope that helps, let me know if you have further questions.

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