This alone does not make the tactic invalid. A glowing review at least means that the candidate was able to make friends which is still a plus in the context of

This alone does not make the tactic invalid. A glowing review at least means that the candidate was able to make friends which is still a plus in the context of hiring. The real danger is that the person you called is unethical and feels the need to get even for things like the candidate having exposed their own incompetence - clearly not a minus in the context of hiring.