Or how 'bout you tell him to read the Magnusson-Moss Warranty Act? The burden of proof is on THEM, not you. Another option would be for him to learn how an automotive electrical system works, that way he can avoid saying foolish things.
I hate hearing stories like this. Google Magnusson-Moss so you can get an idea of what your rights are. As far as looking for proof to back up your claims, check this out:
https://www.google.com/search?q=370Z...ient=firefox-a
Print some of those out or just ask him to go Google it next time you're in.