Diverticulitis-foods to avoid

My husband who is a type II diabetic, recently had underwent surgery for colon cancer which was found during what we thought was going to be a routing colonoscopy. It was also discovered that he has diverticulitis. The dietician at the hospital came in and advised him to stop eatting nuts, berries and anything with seeds. I would really like to have this clarified as the more we think about it, many things in his diet has seed.