In "Of Mice and Men", why is Candy confident about Curley's wife's threats to have them canned?
You are using a confusing term here: "confident".
Do you mean "why does Candy take Curely's wife threats to have them canned seriously?
If so he has a lot of reasons to.
For one, she is the wife of the boss' son. In that capacity she has the power to make accusations. Curley, her husband, is extremely volatile and fights with anyone if given the chance. He is constantly worried that someone at the ranch is sleeping with his wife. He even accuses Slim of sleeping with her. In this case, if Curley's wife were to make this sort of accusation, the men would definitely be fired.