Well firstly they can't show me logical, mathematical and empirical evidence of God because there is none.
But it would sound to me like they have taken psychedelic drugs. Or something like that. The brain is very imperfect, it doesn't have the precision of a computer hard-drive and can malfunction when on drugs, or if you are just generally feeling unhealthy or for all kinds of reasons. Or maybe they are just lying. Who knows?
Why does God reveal himself to the stoner but not to the many thousands of very normal people who live their lives well and have never claimed to see God? God must have a strange sense of humour.
And how does having a personal experience of god explain people from other religions who have claimed the same thing about their god or gods and don't have a psychiatric history? I guess they were just hallucinating?
I think that what can be proven you cannot know, and what you can know cannot be proven. This is because absolute knowledge is obtained through direct experience (not an experience 'of something,' but through experience, period) while 'proving' requires we reach indirectly to 'evidence' that is abstractly interpreted.
If I've felt the warmth of the sun on my face, I can't prove that to you no matter how many thermometers I find or no matter how many times I tell you I felt that warmth. Ultimately, you need to trust that either I'm not lying or that thermometers are an objective measure of something subjective such as a warm feeling. The experience of warmth is something that is known but cannot be proven.
As far as "Why does God reveal himself to the stoner," I can tell you that the Bible says, "Seek and you shall find," but note that it does not tell you what to seek. It simply says to 'seek.' While I denounced Christianity long ago (I was raised Catholic, denounced it and became a strict atheist, then eventually denounced atheism after I experienced it to be false), my experience resonates with several Bible passages, including this one. Because of my experience in meditation I understand the power of "seeking" without having a predetermined object of search in mind. If you observe purely and unbiased, such as in a meditative state where experience clearly diverges from the abstraction of it, absolute truth will reveal itself to you.
Every photographer knows that to get an image with the most clarity, he must be totally still. If he's bouncing around or moving while trying to take a picture, the resulting image will be blurry. You need to learn to be like a good photographer, to still your mind and to observe with single-pointed concentration. If you do, everything will become much more clear to you.