well if he's interested may i suggest two books that will introduce you to the history and culture of these drugs.

The Doors of Perception by Aldous Huxley (this is what the The Doors named themselves after) is about discovering mescaline when it was legal and given to test patients in clinics. Huxley used it heavily himself and wrote theory that will completely blow your mind.
and...
The Electric Cool Aid Acid Test by Tom Woolfe follows the story of an award winning writer's journeys into the realm of LSD, also when it was still legal. This is where the hippy movement STARTED, no joke, these guys where the ORIGINALS!!!! (see also Timothy Leary

these drugs are defintely spiritual, but not like church (bull shit) spiritual, I mean like throwing your mind out into the ether spiritual. The only way to really understand is to do them.