Welcome to the Community forums! This is a great place to get assistance with all sorts of Slingbox questions. In your case, since the router in the setup configuration was changed, you would need to reconfigure the unit from the home location. It's a pretty simple process, and can be done either through the TV on-screen interface using your physical Sling remote, or through a computer connected to the same network as your Slingbox 500.
You mentioned that the unit is located at a friend's house. He would want to hard reset the unit and go through the setup process on the new network. Here are some helpful support articles that will assist you:
Hope this information is helpful!
Sling Media Technical Support
I'm new to the community, and have a quick question. Does anyone know if there's a way to switch the network connection on my Slingbox 500 from WiFi to Ethernet remotely? I'm 500 miles from it, and can't find a way to make this change thru the web interface. It's at a friends house, and he had to change his router, which in turn changed IP's, etc...