I would recommend a couple of things.
1. Have your friend use one of the Slingplayer clients to see if he can connect to the Slingbox locally. This will ensure that the box has an IP address and is still functional.
If he cannot connect locally, try a factory reset of the Slingbox http://support.slingmedia.com/get/KB-1000038
2. Have your friend check the Slingbox's current IP address and port number, and make sure it matches the port forwarding rules on the router. If your Slingbox isn't configured with a static or reserved IP address then its possible it now has a different one. This would invalidate the current port forwarding rules of the router.
From the Slingplayer Desktop for PC, click on Help in the command bar | About Slingplayer <model> (Alt+E) | Information tab and in the LAN subsection you'll see the information for "IP Address" and "Port"
If walk through instructions in needed, he can use the guides at http://portforward.com/english/applications/port_forwarding/SlingBox/SlingBoxindex.htm
That's it. Complication turned simplified.