Err you will most likely need his IP and I mean the IP assigned to him by the broadband company as well as the IP he is automatically assigned to if he has an internal home network. You will then have to have you friend go into the router and specify that anytime someone tries to access using the IP given to him by the ISP then it will route directly the request to his computer. I think this is called setting up the DMZ and all you gotta do is set it up in the router.
There is another service called
www.no-ip.com or something like that where for free you get a choice of a url which will act as an alias for your friends IP. So instead of typing xx.xxxx.xxxx.xxxx you will say friendscomp.no-ip.info in the url and it will take you there. These are some of the ways off the top of my head. I am kind of in a rush so I am typing this as fast as I can and so i do apologize for the lack of details.