Rescue Ninni by talking to her. She is on the large central island. To reach her, cross the bridge to the central island, walk cautiously to the SE until you see her. She is hanging off of a cliff. A good jumping off point is ( -30, -387, 48 ) /waypoint -30.21, -386.98, 47.90.
Talk to Ninni some more. You can carefully walk back up to the connecting bridge (or save your escape for this moment).