PVC, whether Sch 80, Sch 40, or any other type, is not approved for hot water due to the very reason you stated. On the other hand, CPVC is approved, but not my material of choice. A brass nipple should have been used. Short of that a 1/2″ piece of copper cut to length with male adapters sweated on each end also works. In any case, you home builder is full of it and is trying to weasel out of accepting responsibility.