It sounds like you have taken admirable steps to intervene. I'm so sorry you are going through this, and my heart breaks for your son, to be this sad, and yet to be so willing to resume contact with this girl.
I live in Massachusetts and the case mentioned below is all over the news. Here's just one link: http://6abc.com/news/prosecutors-teen-girl-encouraged-boy... The trial is ongoing now.
I understand that you feel you should not prevent your son from seeing this girl, lest you drive him closer. That may be true, but you might consider blocking her texts from his phone. If she has anything to say to him, why can't she say it to his face? Mostly, kids won't do that - they prefer the anonymity of texting, and often say all kinds of things because they are protected by the cyber barrier.
I guess I would suggest you explore this - if this is a criminal issue (as evidenced by the girl being on trial in Massachusetts), isn't it potentially criminal where you live? What action would you take if you observed other criminal behavior or threats in any other area? How would you feel if another kid committed suicide due to her urging? I suggest you discuss this with the police. Your son does not have to know - and should not know. If she is encouraging other boys to do the same thing, she needs at least as much therapy as your son. Maybe she needs charges filed against her. Since these actions occurred outside of school, it's a police issue/DA issue and up to them to decide what needs to be done. A visit from the police to her or her parents might make a difference and convince them that this is not harmless. I suspect a discussion between the authorities and the school might convince the school of potential liability here for doing nothing.
Meantime, of course your son needs more help since he values her life more than his own and so on. Please take whatever actions your counselor recommends, and just as with medical issues, consider a second opinion to get an agreed-upon course of action. You are your son's greatest advocate and the only one making rational decisions right now. Do what you have to for him to be protected.