I've had IBS for nearly 15 years and have tried a lot of different diets. Everyone is different, but two things that come up repeatedly for IBS sufferers are fats and insoluble fibers. Fried, greasy food is the obvious one, but even healthy fats can cause discomfort. Likewise, I loved the idea of a raw food diet, but it just about killed me. Insoluble fibers can be a equally difficult thing to digest. It's ironic, because my own leanings are toward a diet high in healthy fats, whole grains, and lots of raw fruits and vegetables, and for someone with a normal digestive system, that would be a great diet. But for IBS, sourdough breads and white breads (rather than whole grain), cooked fruits and veggies, and low fat foods have made a huge difference in my symptoms. And I hate to say it, but caffeine and alcohol are absolute nightmares when it comes to IBS.
It helps not to eat within 2 hours of going to bed, and to take a walk or do some mild exercise after eating to prompt the digestive system into working a bit more. Ginger (which comes in capsule form) can help smooth out the muscle fibers in the digestive tract and relax them, so that you digest foods better and have fewer symptoms. A prescription for naprosyn (a fairly mild pain reliever that also targets the muscles of the digestive system) can help, as well. Daily probiotics and digestive enzymes can be good, as well as supplemental calcium and magnesium (the magnesium especially right before bed if IBS gets in the way of sleeping). Some people find that soluble fiber supplements help with regularity and general comfort -- keeping the digestive tract at about the same level of "fullness" rather than going "full-empty-full-empty" can keep those muscles working better when they don't work so well under the normal rhythm of digestion. Mints are stomach-soothers, too, and socially it's not obvious that you're having an IBS flare-up when you pop a mint, so he might be more comfortable carrying a tin of those around rather than something like ginger or calcium supplements when he's out and about, you know?