Eating cactus dust soaked in xylene? So he just placed the cactus dust in the xylene and then evapped it all and took it? Sounds strange.
For the xylene to be able to absorb mescaline you need to have the mescaline available in a freebase form. Mescaline, when it is inside the cactus, is in a salt form, and a salt form of mescaline will not be absorbed by the xylene.
You make a freebase out of the mescaline-salt by basing it. So yes, you are right that you need to base the cactus to make the the mescaline move into the xylene.
Kind regards,
The Traveler