If your conciousness did slip into another universe, then wouldn't your brain flat-line.
If we are to believe that there is a coorolation between brainwaves and conciousness, then the precese of one implies the other or the absence of one implies the other.

When you are in a coma, you are not brain flat-lined.
When you are asleep, you are not brain flat-lined.

The best arguement for cross universe travel would therefore be a case where a person is in a true vegetative state (brain flat-line) and they come back and can relate experiences. This would show that conciousness is not tied into brain waves and therefore is tied into something else.

So it boils down to your story teller using vegetative in the right sense of the word (brain flat-line) or in the wrong sense (merely unconcious or comatose). It boils down to having a brain monitor on him the whole six months or merely sampling (so that it's sometimes flat but sometimes active)

Bottome line however dude is I would not trust my universal worldview on the story of one person. And look up occam's razor when it comes to logic: multiple universes is not the simplest explanation and thus according to the razor it's likely not the right on.