I think "in phase" is the correct way of describing this. Correct me if I'm wrong.
The situation is this: You group with another player to do a quest. You discover that one player has done the quest, and you may have started it and not finished it, but the player in your group is invisible except for the white arrow. The logic is that the world has changed a bit for the player that has completed the quest, but has not changed yet for you.
Ok. You say let's find a quest that neither of us has done before and start fresh. What is the best method for restoring your status as a group in which each player is visible to the other?
BTW, I personally wish there was a way for a player to "revisit" a quest that he or she has completed so this type of disruption would not occur. I successfully revisited a World Boss that I had previously defeated with a group member who had not encountered this boss, and it was quite fun and successful. I know World Bosses do not have an impact on the overall story to a zone, but it seems within the realm of possibility that a player (grouping with someone who had not been given a particular quest) could reenter that "phase" or "instance" temporarily while grouped with that player who is new to the quest.