Open up the right ports.

Than just session_open and let him session_connect with your internet ip (unless you two have a lan connection). (check whatismyip.com or whatever it is called)

The more professional way is to get a website that saves server info and create a server list window that gets that info and shows it to people who want to join something. This would also let you names of servers, number of people in it etc.

Oh and be sure nothing blocks the game (like some antivirus software).