If he's still in-game, do: "status" in the developer console to see the list of every User and their Steam ID, or spectate him and get his SteamID64 or whatever it's called.

Good luck!

If he's still in-game, do: "status" in the developer console to see the list of every User and their Steam ID, or spectate him and get his SteamID64 or whatever it's called. Good luck!