Phil,
If all the lights were green, it means the flow switch was working and from the cell's point of view the salt level was ok..
The most likely cause of the cell "not keeping up", is algae...
It could be that the actual salt level is low, as Allen has suggested, or the cell could be on its way out, but without test data we are all just guessing.
I understand it is not your pool, but the best thing to do is have your neighbor run an OCLT... with the SWCG off.. If the OCLT passes, I'd run another OCLT with the cell on at 100%.. This will tell you how much chlorine the cell is making per hour run..
Thanks,
Jim R.