From an HR standpoint, the determining factor here is not whether the vaccinations are actually required. The factor that determines whether there's any legal issues here is whether or not he was fired BECAUSE he pointed out what he believed to be an illegality to his employer.
I am by no means convinced that anything illegal transpired. But I am by no means convinced that nothing illegal transpired, either. It can do no harm to discuss it with corporate and/or bring it to the attention of the appropriate regulatory agency. The worst that can happen is nothing. He's already lost his job.