IBS can be a very serious problem for some people; kind of like having a constant stomach flu. It can cause severe diarrhea, excrutiating stomach cramps, and nausea. All of these symptoms have a profound impact on a person's life, obviously. Also, keep in mind that many people with so-called IBS actually end up having more serious conditions, such as inflammatory bowel disease, and were simply misdiagnosed.
I myself have had chronic diarrhea for a year, up to 30 times a day, and this has wreaked major havoc on pretty much all aspects of my life. Luckily, I have wonderful employers, and FMLA has saved my life.
I encourage you to provide as much support as possible for your employee. She may be undergoing more medical testing to elucidate the cause of her symptoms and it's likely a very stressful time for her. I think it's great you are educating yourself about this very mysterious but often horrible condition.