Usually doing the press & hold power button for 20 seconds and waiting for the Kindle to reboot with the boy under the tree screen is all it takes to 'encourage' the wi-fi to rescan, but you could also perhaps try rebooting your router too. Another thing if all else fails is to try is to reset your Kindle to factory default settings, but you should copy all your books from the Kindle first before doing this.
In terms of your friends router, a quick search shows connection problems with the Virgin Superhubs, looks like you have to fiddle about with settings on the Superhub before the Kindle will connect to it. First thing is to try and change the security settings on the hub to WPA2, and if that doesn't work you've got to change a couple of other things, see this thread on the Virgin Media forums for an example:
Virgin Media - Kindle connection to Superhub 2