Once Montag knows the truth, he is desperate to expose the fallacies of his controlling government and chooses to share the truth with his wife. Mildred is a poor choice for this knowledge for several reasons:
She is not emotionally mature. When Mildred learns of the books, she exclaims,
"Books aren't people. You read and I look all around, but there isn't anybody!"
He stared at the parlor that was dead and gray as the waters of an ocean that might teem with life if they switched on the electronic sun.
"Now," said Mildred, "my 'family' is people. They tell me things; I laugh, they laugh! And the colors!" (part 2)
While she cannot fathom the possibilities of books and cannot respond to them intellectually or emotionally, she longs for the stories and "family" portrayed in her parlor. In fact, this false sense of reality is all she longs for, and her life seemingly revolves around it. Later, Montag presses her:
"Millie? Does the White Clown love you?"
"Millie, does"—he licked his lips—"does your 'family' love you, love you very much, love you with all their heart and soul, Millie?"
He felt her blinking slowly at the back of his neck. "Why'd you ask a silly question like that?"
Here it becomes clear that Mildred cannot emotionally connect with her husband and instead hinges her emotions on her false sense of reality, often brought to her through her fictitious "family."
She also wants to save herself. When Montag brings his books to the attention of Millie's gathering of friends, she is shocked. Millie tries to shush him and even tries to snatch his books away. She knows the possession of books is a dangerous business and even concocts a story about how each fireman is allowed to bring home one book a year to share with his family so that they all realize how silly books are. Considering her relative difficulty in navigating anything beyond superficial conversation, this shows the level of fear that Montag's display has stirred in Millie. Millie knows that too many people now know Montag's secret after he frightens her friends with poetry, and she wants to be sure she isn't implicated. One certain way to do this is to turn him in to the authorities herself.
Millie is not a hero. Throughout the novel, she makes no bold moves. She has no deeply introspective thoughts. She doesn't actively support her husband's passions. She is passive and does exactly as the government has conditioned her without question. She is content to live a life of (what she perceives to be) comfort and to be left alone with her vapid friends and false reality. She turns in her husband because he longs for more, and he threatens her sense of comfort and societal norms by doing so.