You seem to be invoking an instance of the Sapir-Whorf Hypothesis, which has been soundly disproven. There is something to be said for the approach of a therapeutic presence with people with mental issues, but a becoming-insane is not a necessary result of it.
I don't think he meant it literally, as in acquire a mental illness, but to "go along with it" for a little, in order to facilitate communication.
The sufferer's world-view, insane as it may be, still has some form of internal logic and structure, large parts of it will be very twisted, but as the sane mind of the two, you have the ability to step into theirs for a tiny bit, see it from their perspective (they themselves are either incapable or tired and worn out from constantly having to adapt to a world-view that doesn't make sense to them). Your advantage is that you only have to bring up the energy to make translate a crazy world-view for a few hours, their plight is 24/7.
(also, the Sapir–Whorf hypothesis is about linguistics, I don't think you can naturally extrapolate what has been proven about it to the field of mental health)
