 Why was the girl unable to help Joe commit his planned crime?
I've been a garbage man, street-cleaner, ditch-digger—" On the other side of the room, the jukebox came to life with a roar and a group of teen-agers scrambled to the dance floor. Feeling safe from hidden microphones because of the uproar, he leaned across the table and whispered in the girl's ear, "That's what I want to hire you for. A civilization weary of murder, robbery, kidnapping, counterfeiting, blackmail, rape, arson, and drunkenness had originated the CPA—Crime Prevention Association. There were no longer any prisons—CPA officials had declared loudly and emphatically that their job was to prevent crime, not punish it. Instead, he was sent to a hospital where all criminal tendencies were removed from his mind by psychologists, shock treatments, encephalographic devices, a form of prefrontal lobotomy and a dozen other methods. He was a hero to the millions of men and women who had suppressed impulses to kill someone, beat their mates, get drunk, or kick a dog. The girl followed him across the room, around tables, through a door, down a hall, through a back door and into the alley. A crime because it theoretically inflicted psychological injury upon the intended victim—and because millions of women voters had voted it a crime. On the other hand, attempted murder, robbery, kidnapping, etc., were not crimes.
 She was a part of the WSDA and obviously knew how to defend herself.
 She was a part of the CPA and didn't agree with his idea.  She was unable to accept his small payment for a large possible consequence.  She was a part of the WSDA and obviously knew how to defend herself.  She knew no one would believe that he had actually tried to rape her because of her status as a DCT.