How did they get him to the site and how did the know his handle?
I just don’t understand the girls relationship to her counselor. Drug program?
That's easy. He (allegedly) went on his own because he likes young, barely-legal girls. They either knew from a PI following him and then questioning a previous girl on her way out, from hacking his computer, or from getting access to his ISP's information.
Anyone who wants to take someone down can get the information, if that person steps outside the lines. It just takes money. He's not high profile enough for unsubstantiated allegations from high school to hurt him, but the problem for him is if he really did what is claimed (whether or not he would have done it if he had realized she was underage).