Who was the informant on Montag's home?

In Part III, Montag finds himself at his house with Beatty and the firemen. Beatty is lecturing him and places him under arrest. Montag supposes that Mildred saw him hide the books in the garden. Finally, he asks Beatty if it was Mildred who turned him in and signaled the alarm. Beatty nods that she did, but adds that Mildred's friends turned him in first. Beatty scolds him again for quoting poetry "free and easy...

In Part III, Montag finds himself at his house with Beatty and the firemen. Beatty is lecturing him and places him under arrest. Montag supposes that Mildred saw him hide the books in the garden. Finally, he asks Beatty if it was Mildred who turned him in and signaled the alarm. Beatty nods that she did, but adds that Mildred's friends turned him in first. Beatty scolds him again for quoting poetry "free and easy like that." This shows the extent of repressed freedom in this society. Montag could not even quote poetry in his own home. His mistake was doing so in front of Millie's friends.  

Recall that in the previous section, Montag decided to have a more meaningful discussion with Millie's friends. First, he brings up politics. Then poetry comes up and Montag reads "Dover Beach" by Matthew Arnold. In this poem, the speaker talks about how the world is changing and how the future could be wide open and wondrous. Millie's friends are overcome with emotion and don't know how to handle it. Montag is trying to get them to feel something real, rather than the passive pacification they get from the parlour shows. To drive his point home, Montag particularly scolds Mrs. Bowles, trying to make her think about the real emotional experiences from her real life: 

Go home and think of your first husband divorced and your second husband killed in a jet and your third husband blowing his brains out, go home and think of the dozen abortions you've had, go home and think of that and your damn Caesarian sections, too, and your children who hate your guts! Go home and think how it all happened and what did you ever do to stop it? 

It is quite possible that Mrs. Bowles turned him in after this tirade.