Could be a high efficiency gas furnace. They produce condensate. Or he could have a humidifier. We had trouble with my parents one that we discovered a week ago during the fall service. The GFCI breaker on the pump tripped. We turned it on and it ran the pump but didn’t drain. The service tech thought it was a bad pump, but the line was clogged even though he put tabs in to prevent it in the spring. The condensate pump is for the water heater, humidifier, dehumidifier, and evaporator.