If your friend is not in the same network as you are, your Dedicated Server needs to be set to Internet not LAN. Secondly, when you give him the IP, make sure that it's the public IP, not the internal one of your computer. You might need to forward the 27015 port to the IP of your computer. Secondly, there was a bug in Counter Strike that prevented you to add servers to the Favorites. Make sure to bring down the console with ~ and type connect IP:PORT and it should work.