"Connections: Xfinity DVR into TV, ARC out to receiver"
What if you go the traditional route of cable box at the AVR? You could still use some of the CEC stuff if you wanted.
I would guess the issue is the channel and or show(s) they are watching. What seems to be popular with some folks is the crappy local feeds showing old reruns or the old imported BBC shows on PBS.
"There's a big difference between winging it and seeing what happens. Now let's see what happens." ~MacGruber