Er.. tell him your IP number (something like 207.328.43.345) in MSN instant Messenger... (Run winipcfg in startmenu if you don't know your IP)
Then tell him to go to Play>Join Game>New Favorite>type your IP, then double click on your server name.
Or tell him the IP of your favorite server or an empty server you want to duel at.
Don't run games from your instant messenger, they are a bit iffy with the latest games.. I'm sure they can start Hearts games real well