In normal servers when you disconnect you are removed from the world, so mobs can't attack you. However they will stay there (unless removed because no one's been there for a while I think and the chunk gets unloaded).
If there are plugins installed the guy can use one to keep the player "logged in" for x seconds after signing out, usually meant to prevent log out cheating in PvP. Such a plugin would leave you sitting there to get beaten on by the mobs until you died most likely.
I'd ask the guy to stop spawning them as it is ruining players' fun and causing them to leave. If he doesn't then find a different server!