I know absolutely nothing outside of what you stated in the post so I will only comment on what I know from that. The girl is 17 which happens to be the age of consent in your state so regardless of your feelings on a relationship between a 40 year old and a 17 year old it’s not illegal as you have stated yourself. I saw in another comment that the family of this girl did not want to pursue this any further, however you still feel entitled to disrespect their wishes and pursue it anyways?
Finally I want to add that this post makes me think you have it out for the black belt. You go out of your way to make the male out to be as creepy as possible by adding in a useless story where you thought he was creepily flirting with a 16 year old but you have no proof. Then you go on to express that the girl is young looking and naive, making her out to be the innocent one here. My point is there is some clear biased against the male subject you mention and that doesn’t bode well for you in a legal case of defamation. Especially as you are unable to prove he has done anything illegal. I would recommend you stay out of the situation unless you have undeniable proof of something illegal. From what you are telling us it doesn’t sound like that is the case.