friendly fire in HL2:DM

I am trying to make a "survival" map where players spawn in a building and are bombarded with endless waves of combine. The only problem is, I dont wanna have to deal with idiots killing the other players since this is a pseudo-coop map. So I was wondering if theres a way that I can make players invincible to each other but still take damage from npcs. Kinda like the way the resistance soldiers are in HL2 single player. The only catch is, I can't mod....

Any help would be appreciated

Though I haven't tried it, I've read that (almost ?) all npc AI is
disabled in HL2DM, so you won't be able to have npc combines attacking.

Sounds like a mod coding job, rather than a mapping problem.

arg

If that is the case, is there anyway I could tweak the map to allow for internet play through HL2, I know theres a way to connect to other peoples comps through the console, but it never works, I always assumed it was just because single player maps didnt have spawn points for the second player.....

You might wait for the Sven Co-op
team to release for HL2, and produce a map for that, since that
mod is essentially exactly what you want to do. Of course, there's no
set release date, and these things always take time.