I have a recipe that calls for a Cubanelle Pepper, but my dad can't eat peppers.
Note that Cubanelle peppers are not hot peppers — they're similar to green bell peppers. So if your father's restriction is restricted to hot peppers, then he'd be just fine. 

Otherwise, there's nothing else quite like the flavor of peppers (other than paprika, which of course, is made from peppers). Your recipe has so many other flavorful ingredients, that it's sure to be delicious if you simply leave out the Cubanelle pepper.

However, celery might be a good substitute in your recipe, being similar in color and texture to Cubanelle peppers. The flavor of celery is subtle and would work quite well with the other ingredients you listed.