The only source I see is a tweet from this guy-
https://twitter.com/JGrega11
I can't find anything else confirming or denying that McMurphy said it. The only other thing I have found is the possibility that the quote was originally from Grimes' father, and just wasn't attributed properly when McMurphy relayed it.
BTW- Not sure why you have such a hard on for McMurphy, but this is not looking good for him. Zack Smith is a crazy p.o.s, but, as Spock has pointed out from the get go, it's entirely possible that his wife is too. McMurphy's latest effort doesn't do him much good in regard to the original story.