moonkissed - sending you my best supportive thoughts as you deal with a difficult situation. I think you're right on
feeling that something isn't right here.
I took this question to someone who knows what Child Services would do - although laws and rules differ in different states.
Child Services would only act if the adult was a guardian of the minor. Otherwise they refer the complaint to the District Attorney. There isn't much to go on for the DA to take action
The legal issues are not clear about whether Child Services could look at a Facebook page without the author's permission. Where "not clear" means exactly that; opinions differ about the legality of that particular breach of privacy.
This is a situation for an adult to help the minor; if she doesn't have someone at home, is it possible to contact her school counselor or school nurse? Seem like she might just need a little guidance, a little chat about what could be going on in the mind of the older guy. Some 16 year olds might be shocked that he might be thinking of THAT.
You're a kind soul to be concerned and willing to take action. I do hope you can identify an adult female who can give the young lady some time.