"the language centers of the speakers’ brains decreased during glossolalia"

This is particularly interesting, because it exhibits the same effects as entheogens which, too, decrease brain activity (contrary to popular belief). It would make sense since glossolalia as you describe it seems similar to a (self-induced) trance. It would be interesting to know if your friend ever heard that particular prayer he recited. In my experience, our senses (and memory) are much more sensitive than we are aware of in our standard "waking state" and trance-like states make the gates of unconscious wide open.