How many wires does your flow switch assembly have? Old ones, with just the flow switch, have only 2 wires. For the past few years they have had 4 wires.. Red and Black for the switch and Green and White for the Temp thermistor. And I have heard that the very latest one only has 3 wires, but I have yet to see one of those..
A neighbor said that his SWG wasn't keeping up and that all the lights on the display were green.
He brought it over and when I took off the cover for the flow switch I noticed that someone previously replaced the flow switch. I also noticed that the white wire had come out of the connector. I'm not sure if I had pulled the wire out when I removed the cover (it was a tangled mess). I cleaned up the wiring and was hoping that any production problems were related to that, but he said all lights were green before he brought it over.
The flow switch was after market with 4 wires.
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..