That's a great way to make them lose their trust in medical professionals indefinitely. It's probably not a good idea to reinforce their delusions, either.
Fair point, and I don't mean to endorse the quote as psychiatric advice (nor do I believe the quote was intended as such). I took is as an amusing expression of a general principle, that people with deluded beliefs may be quite rational in following the consequences of those beliefs, which should be taken into account when dealing with them.
Rationality quotes time!
The usual rules: