I wish I had words of wisdom for you. Teenage girls are tough to parent! (At least my experience is )
I try not to preach to my girls ( I have one 15 and one 19) but I do try to plant seeds as they say every chance I get - whether it is about how important friendships are, sex, boys, whatever. I know it won't do any good to try to control who they spend their time with as they will just work that much harder at doing the opposite. I guess as a parent, we just have to bet there for them, and hope that we have taught them enough right from wrong and continue to encourage them to make the right choices.
I'm going through my own struggles with DD 19 so I understand. hopefully she realizes that this boy will most likely not be her "forever" and learn from her experiences.
not much help, sorry. hang in there.