TCP sockets, but you need to know the other player's IP. You'll need to have a master server that stores the IPs for players by names.
Have the mod make a tcp socket listening for 1 person on a port, then have the person sending the message's socket connect to the ip you found on the server and send the messsage, then have the receiving socket handle the message and chat message what was received.
Not as easy as you thought I bet.