How exactly is nurse Ratched bad?
I don't see it, I see a nurse that trying to uphold the rules. She was absolutely right that altering the schedule would result in confusion in the patients who are mentally ill and are used to it.
And after the big party, what was she to do, sweep it under the rug and let them get away with it without repercussions? We love and root for Billy, sure, but she's in a position of authority who has a responsibility to the patients under her charge.
Allowing Billy to have sex with a prostitute on hospital grounds would be shirking her duties and would cost her her job.
I know we're supposed to be on the side of the free spirited McMurphy and the rest of the guys but in reality nurse Ratched ran things the way she needed to.