Just ask her about it. It's kind of an awkward subject, but if you're really going to be serious with someone, you need to talk about stuff like this. Be sure to be absolutely free of judgement. People have all kinds of weird body issues. IBS isn't actually that bad for most people if properly managed. Gluten is often the culprit. Dairy messes up a lot of people's digestive systems too. My symptoms probably wouldn't have been bad enough for a medical diagnosis, yet I still started feeling much better when I eliminated both gluten and dairy. Even though I suspect it's just a hippy fad for a lot of people there are a lot of people who are truly "sensitive" to these gluten and dairy.
Having a healthy GI tract is actually super important. If your GI tract is inflamed or messed up, you're not going to be absorbing nutrients very well, and it can cause poor systemic health due to nutrient deficiencies and other things. I would try to nudge her to get this figured out.