How exactly does his ISP's decision to keep broken DNS become your problem?
Simple -- he'd make the case that you should only refuse to deliver mail if there are no correct MX records, not if there are any broken ones.
And I'll make a case that broken DNS records are a sign of an incompetently-administered ISP, and that it's been historically shown that incompetently-administered ISPs typically have other problems, such as open relays and hacked proxies, and zombies.
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