I think in that case it falls under mandatory warranty rules - if he didn't replace or reimburse them, he'd be liable. I don't know if warranty rules in the US are as strict as in Europe though. But in Europe, if your product is malfunctional within a year or three years (it's three for electronics, which is why warranty extensions from 1 to 3 years like AppleCare are dodgy), you as manufacturer are responsible for a replacement or compensation.