MIldred's friends, who had been at Montag's home when he read poetry to them, had called in a report prior to Mildred's. However, Beatty ignored (he does not explain why before Montag kills him). He knew it was a matter of time before Montag would be inextricably caught. He knew Montag's personality well enough that Montag would give himself away through sheer pride, frustration, or self-destruction. He didn't count on Montag's true commitment to this new rebellion.
Mildred is the informant on Montag. Although she has been reading with him and letting him read to her, Mildred's mind and consciousness can not be penetrated. She believes firmly in the "social order". She doesn't want to think. She is happy just to exist. Of course, this is an oxymoron - for she isn't happy and she doesn't really "exist." Mildred despairs, finding no pleasure in life and seeming to not want to find pleasure. Montag's new pursuits scare her.
It's perpetual motion; the thing man wanted to invent but never did. . . . It's a mystery. . . . Its real beauty is that it destroys responsibility and consequences . . . clean, quick, sure; nothing to rot later. Antibiotic, aesthetic, practical. - Beatty to Montag, just before Montag kills him.