Seems pretty straight forward actually:
Girl uses fake ID to sign up for the website, and goes out and does the deed.
Either she was already talking to a therapist, or she starts going to, and shares the story of her foray into prostitution.
Therapist says some version of "You know I'm a mandatory reporter for underage sex crimes, right? I have to call the cops now."
Girl didn't want any of that.
Therapist calls the cops.
Girl tries to contact the guy she wasn't trying to get arrested to warn him.
Drama ensues.