McMurphy has observed that the patients have been controlled for so long that they do not make any decisions for themselves. The process of giving the men their manhood back does not happen overnight. It is based on a series of events. They observe McMurphy stand up to Nurse Ratched and that he is able to get under her skin enough that it is appealing to them.
McMurphy basically teaches them that the world outside has great things to offer them versus the world inside. He gets them on an illegal outing fishing and does things with them that they would not undertake for themselves. Gradulaly, they begin to step out of their comfort zones.
When McMurphy throws the party and sneaks women into the hospital as well as alcohol, Billy experiences a relationship for the first time. He feels confident and manly. Nurse Ratched is aware of Billy's psychological discomfort and trauma in relation to his interaction with his mother. She brings up that she will share the details of his misbehavior with his mother and that it will be upsetting for her. Billy kills himself as a result.