Right, I got beyond this bit. There are a number of different ways this can glitch and Ill try and cover them all below. Either way - its glitchy as hell but perseverance will get you your keep (Im PS4 in case this doesnt work elsewhere).
The bad news is that there isn't much to do here, you have to leave, progress the story and then come back. You cant fast travel out of the map entirely, but you can fast travel to another area locally and then fast travel again from there.
Go to the right of the entrance and there are some windows that allow you to see into the room. You cant get through them, but if you just press up against them one of your companions will randomly teleport into the room. Switch your control to this team mate and voilà, all your people are in the room.
Kill the monster, ignore the human. Once the monster is dead you can talk to the human form and he will just disappear. Its worth pointing out that just because you got this far and are fighting the monster again there is nothing to prevent him repeating his disappearing routine - In which case just rinse and repeat..of course this time you save right outside the room though right....