Panels are normally warranted not to drop below 80% of rated output after 25 or 30 years, although some are defective and crack within a year or so. But they don't start dropping off more rapidly at that point; rather, their output declines more slowly after that point. Those panels will be fine 50 years from now if he doesn't throw them away.

Inverters are a different problem, and they do in fact die after a few years. But you don't need the inverter to use the energy.