It will not hurt it due to no light. The low light is actually better for keeping it calm.
In your situation, I would first try removing the sailfin into the sump for a week. Feed both the tank and the sump heavily. You need the sailfin to chill out from what it sees at "his" territory, as well as get the angel comfortable and get a lot of nutrition into it. Reintroduction starts in an Acclimating box. With a fish of that size, I would And the box and put the fish in after lights out in the evening, allow the entire light cycle the next day to run. Normally observing both fishes behavior. So long as the angel is still feeling comfortable in the tank with the sail fin present and eating, I would release the sailfin from the box that evening after lights out. A fish that big cannot (should not) stay in an activation box for a few days.