The authentication is just referring to an option where you can setup your character so that it will be locked to a specific IP. This prevents other players from accessing it (if they happen to know your password).
As far as being black screened, if his install was identicle to yours and he made no changes, then that's pretty inexplicable.
If that's the case, I would try to reinstall the game and the mod. If it still doesn't work, we'd have to figure out if it's tied to the server, or his particular install of Tribes. The server can do things that will black screen a character, but I've also seen it where it's tied to a particular install of the game.
After he reinstalls Tribes, and if it's still black screened on PCRPG, try going to a different Tribes server (non-RPG). If it's still black screened then we know it's his Tribes install. If it works fine, then we know it's the server.
