16 girl wants to date a 20 year old man

Our 16 girl wants to date a 20 year old man who dropped out of High School, doesn't have a job, lives with his mother and is not very intelligent. Our girl is in middle college and gets straight A's.

This all started 3 weeks ago. She lied about his age and the nature of the relationship. One week in we discovered the truth and decided that we had to intervene. After speaking with both of them about how it was not only an immoral, but possibly illegal relationship, he was quick to point out the legal age of consent in our state is 16. We told them to wait until she's 18. He seemed rather upset, so the next day I went to his social network page where he had posted the following:
"me plus you equals violence, creepin up behind you I'm stalking you silent, dressed in all black I'm the nighttime assailant, grab you by the neck and stab you with my nine inch, knife, quick like, I end lifes, no strife, i mean i need just one swipe, and then I wipe, your existence away, I'm the evil son, so let me in to play." I also found older posts of murdering people and my favorite" My penis is a weapon".

Though her mother was with me on keeping them from dating, she is now thinking supervised visits is a better option to divert a possible Romeo/Juliette scenario after we discovered that there were over 1400 texts between them in 9 days. Her thinking is the more time they spend together, the more likely the girl will be to discover that the price charming is really a toad. I think a forced separation is in order complete with GPS activated on her cell phone so we know where she is at all times (she has a car).

I am very saddened by all of this because I had a close relationship with her. I have spent a long time explaining why dating him is a terrible idea and that we are really looking out for her and trying to protect her. Since I was the person who told them to stop, I am now the worst person in America. I know after a period of time she will someday be able to appreciate what I am doing, but for now she thinks I am mean and stupid. It would be helpful to be able to give her reason's on why dating a 20 year old man is not a great idea since my reasons have not resonated with her.

I am totally stumped on what to do in this situation and welcome some new ideas.