I'm assuming that they changed their router. If thsat's the case, they need to press and hold the "reset button" on the back of the slingbox to drop all the old settings and rerun the setup wizard to resetup everyting. After completign the setup wizard prceedure including the remote internet viewing section, you should beable to watch again.
Okay, so I will send them to the link below to download the Slingplayer 1.5 application on their home network computer. From there, they will rerun the Set-up Assistant.
Is that it?
I greatly appreciate your help, and will come back with an update as soon as I have some results.