I was doing this with a friend and was on the quest to talk to the person who was trapped in the cage. After finishing the dungeon, I went and freed that person (Forgot the name) and when my friend went to do it she disappeared since it was part of the dungeon for her to disappear. Because of this my friend now has to do the whole dungeon again just to talk to her.
Sorry if this was slightly confusing, it's not very easy to explain when I've forgotten the persons name lol.