What to do when an inspector misses stuff?

Duh! The fact he neglected to look into it is what makes it unreasonable. At minimum he should get a refund from this ***** not knowing what the hell's wrong with roofs. What else did he miss if he can discount this?

What you are talking about is an issue of Equity, not law. Law very rarely cares about Equity and the OP asked a question about law, not Equity.