This just happened to me last night. I was a legionnaire and my friend was a keeper. It was just the 2 of us. He could see everything just fine. I followed him and when enemies were close to me he'd tell me and I'd war cry, walking fortress and just kept swinging. That was zone 3. When he entered zone 4 I teleported back and was back in the map. Then it happened again on zone 5. We somehow survived the venitori commander and were successful, routine difficulty. Then my friend, the host, locked up and had to jump out. I then became the leader and was at the start of zone 5. He jumped back in and we beat em again!
Zundar... Is that you, buddy? We thought we had lost you to the Fade, my friend... whatever you do don't walk towards the light... It's a trap!