Have you replace the brake light on the bike with LED lights? that will stop it from working.
You're kidding, right? Why would that make a difference? Regardless, pretty sure dad is running regular bulbs everywhere on the bike, but I'll check when I head up there to see him.
It just seems odd that the seller would be like "it works" and dad can't get it to work. Is there some sort of special way to engage the system that I should be aware of?