If it is post filter, and if he has a sand filter that is failing, it could be putting sand into the solar panels. Maybe try flushing it, you could remove it from the pool, lay it flat and run a hose through it and see if it comes through clean.
When our pump hoses would get sand in them, the only way to get it out was to pump clean water through them. (Pump hoses, as in pumping ground water with construction pumps)