They aren't left alone. As your quote says "You're with two guides, one male, one female."
The conditions are almost a sure fire way to make someone have a troubling experience if they were given the doses knowen to be taken in those times and did not have prior experience dealing with the reality altering effects of those substances.
Just adjacent to her office was a "trip room", probably constructed when the building was build in the 70s (?). It was round, completely lined with acid orange fabric, upholstered like a pasha's tent and filled with cushions in every colour imaginable. No doubt it had a superlative quadraphonic sound system installed. It also had a large, smoked glass window, for "observers." Not ideal for mushrooms perhaps but for other things...