Personally, I would be greatly concerned. Late teens to mid twenties is the prime time for schizophrenia to start to manifest. Given that electrodes in the brain can simulate religious experiences and such, Im more inclined to believe that most (if not all) such things are just all in the head.
No matter what, I would keep an eye on said person just in case they start showing other signs of mental problems.
Ask them about it! Whether is was real or not it was obviously important enough to my friend that they would tell me about it XD
To be honest when i was 15 i thought i could talk to God. I would pray and hear answers and like they were always correct XD I told a friend and he believed me, later when i realized that maybe wasn't right of me to tell him that i explained to him i've probably just created a voice in my head in desperation for answers and he took it well luckily XD
I still believe in God but i'm not sure if we are able to communicate with him. I still "talk," to him but i know its a comfort thing XD