If you want to take full control of his server, you need to edit his server.cfg in the \cstrike\ directory
you need to add:
where xxx is the password.
once you're done, run the dedicated server.
Go back to your computer and type
then you may execute any command as if you were at the hosting computer.
you must add "rcon" (don't use quotes in the console) before any command.
like
the xxx is your variable and the single x is the number.