what events make the cheif think that maybe nurse ratched is right about mac?

Well, consider the time-line of events near the end of the book,

McMurphy becomes a small legend because he refuses to admit he was wrong, even though it means perpetual punishment with electroshock therapy.

Two weeks later, when he’s back on the ward, McMurphy manages to sneak Candy and Sandy on the ward. They have a major ward party, where they all get drunk and Billy and McMurphy go to bed with their two prostitutes.

When they’re discovered, Nurse Ratched shames Billy so bad that he commits suicide. McMurphy is so angry that he smashes a glass door and assaults Nurse Ratched.

When McMurphy returns from the hospital, he’s no longer McMurphy – he’s had a lobotomy.

Chief smothers McMurphy to death in his hospital bed.