Might have experienced ego-death and got the wrong message out of it
there's an experience that is common to high dose psychedelics, to practised meditation and to various other practises that involve honing the mind - it's commonly called ego death at least when it's reached via psychedelics. It has many different names in various religions and spiritual belief sectors, and across those who tend not towards such thinking it has many other names still.
Essentially though, the experience boiled down to it's simplest: You have what at the very least feels like an experience in which you are becoming one with each and every other part of the entire known Universe. You appear to feel no separation from anyone or anything, dead or alive, future or past, all seem to be one and the same. I wouldn't be surprised if it was a touch of this experience that led to Nietszche's last days being spent running around raving about being various famous past figures - in essence, if you believe the nature of the experience to be true, then it wouldn't be wrong to say that, because you essentially then take on the belief that all beings and all things are one and connected, and so I'm not separate from you, and you're not separate from some guy in Australia and he's not separate from some famous Muslim preacher several thousands of years ago and so on. Sounds a little much to take on, but I'd say the whole thing is way too much to take on for anyone who hasn't had the experience themselves. I've had it a few times in my life, some during psychedelic experiences, others during deep meditation - and so have come to what I believe is a better understanding of the experience over time, but I can understand how someone could freak out and take it in a rather weird way.
If ego death, i.e. the sensation of becoming one with everything/everyone is what your friend experienced, then it's possible he got confused about the experience. It really does feel like you have become the entire universe, but to most this is a humbling experience, i.e. you see that you are all interconnected beings in one giant web of life, rather than you seeing "Hey I became God, that makes me the best out there". To infer the two most logical conclusions of it would be: a) I was tripping, it was an experience I had during a trip or b) If it was real (which should really be something you decide once and only once you've experienced it through some other means than psychedelics alone), then this means I am but a humble part of a much greater larger being than me, all is connected and should be loved equally, and no-one person is above any other.
He's taken on the "aha so I am God/God's son/[insert other famous religious figure]" view instead, and not paid attention to the rest of the experience. It might help reminding him that if he experienced such a thing, for one, he was on psychedelics, and so the experience cannot be verified. Two, if the experience carries any truth to it (which many people, myself included, hold that it likely does, but only because I've experienced it sober as well as on drugs and do not make the connection that it is a drug induced psychoses type experience - but this is a matter of personal belief, and not something to go preaching on others) - then it infers that all are connected and
equal and that he's not some messiah who's seen the truth - many other people all over the world have had the same experience he has, and he just has to focus on integrating it into his daily life.
If he chooses to believe it may have some truth to it beyond the hallucinatory nature of the experience, then maybe rather than running around preaching that he's Jesus he should take up meditation or another healthy life pursuit in which he can reach such experiences again without the aid of drugs, and verify to himself that the experience can be reproduced outside of the world of drugs.
More likely than not though if that were what he'd experienced, you wouldn't see him acting like he did afterwards, though you still might notice him saying some things that seem rather out of the norm, as the experience does shake up your sense of reality, in a way one person might deem good, and another might deem bad.
What it sounds like may have happened is that he had a very strong experience, had a lot of experience around Christianity prior to it, and so during his experience he experienced things that seemed to relate in some nature to Christian beliefs. He probably then inferred his own beliefs on top of what was happening, and somehow or another his experience left him thinking he was to preach the message of God and was some messiah of his own. The only way you'll really find out exactly what happened that made him act in that way would be to ask him yourself, but if he had a psychotic break you likely won't get a clear answer out of him - and if it turns out we're all the psychotic ones and he's Christ incarnate no.2 then he's still going to sound rather psychotic to us all isn't he?
Either way, I imagine as whatever he experienced gets a little further and further from his recent memory it'll start to lose it's sense of importance to him and whatever it was in his experience that make him decide to go around preaching, he might at least see that whether or not he still believes it's a good course of action - it's going to get him locked up in the loony bin and not something he should do for that reason alone if nothing else. So I'd assume he'll start coming to his senses fairly quickly.
As for what he actually took, it's hard to say - LSD itself is a fairly benign substance, it doesn't tend to cause experiences of a psychotic nature or to harm the body or mind - but many other substances are sold on blotter as "acid" that are not LSD itself, and that we have not yet fully researched or understood, some of these substances like the NBOMes have more tendency to cause traumatic experiences both of a psychological and physical nature (i.e. psychotic experiences, seizures, etc). There's no sense trying to figure out what it might have been now though as unless he still has some of the blotter he took and can send it off for testing there's no real way of knowing.
Edit: Didn't notice this thread was already a month old and he's now been released. Well that's good news, and glad he's thinking a bit more rationally now Hopefully things continue to improve and he hasn't had any lasting negative change in his personality due to his experience.
