IR, if someone’s dealing with Crohn’s or colitis they probably should take it a step further and look into the specific carbohydrate diet, and the book Breaking the Vicious Cycle. (There’s a dot com website by the same name.) This is the “Crohn’s” diet. Some people with extreme celiac have found the diet helpful but for celiacs it’s recommended for life, and from what I’ve seen, not necessary for most people.
There is a kosher business in Monsey called Digestive Wellness that sells prepared foods and some basic ingredients, as well as a cookbook. The Weisses, who run it, are very helpful and l’shem shamayim, though there may be cheaper outlets for some of the ingredients, a lot of the products are for advanced dieters, and many people find it simpler and cost effective to make them themselves.