He shouldn't. He also shouldn't be able to interact with any other players. (It is like this in any story related area (i.e. Grotto (Raid), Engine Bay (Beginning), Old Mount (During final part of ep. 1), and Isle Boiler (When completing the storyline mission from Emily))
As for the Lasher fight, it is impervious that you take the Lasher out immediately with the sniper. Otherwise, you dead. :0P