Calendula is a wonderful herb and can be used topically or internally to promote the healing and soothing of tissues. It is thought to have mild anti-inflammatory properties and has been known to be helpful for gastrointestinal complaints. For these reasons I think it would be a great idea for diverticulitis.
The best way to address diverticulitis is to prevent it from happening. Making sure you take adequate fiber and water in your diet is so important in prevention. Adding a gentle fiber supplement with plenty of water and making sure you have easy, regular bowel movements daily can be a significant step to preventing further attacks.