I will speak from my experience as a troubled teen.
The most important thing you can do (and obviously already are) is to be involved, and concerned.
I am only speaking from experience my daughter, found herself pregnant at 14 and although it brought us closer together emotionally, it was not a pleasant experience.
She is now 18, more mature and a fabulous person, looking back she says I was just a kid - what did I know that something like that would happen to me. I have a 16 year old daughter, though she is pretty easy, but I was a troubled 16 year old once (and I have also transracially adopted a child with challenges).
It's unclear to me what your goals are, and this is the first step in setting up guidelines or procedures of any sort.
It sounds from your message as if you are conflicted with regard to your goal.
I guess that is what I would most emphasize: share with her your genuine concern that she make choices that support her and tell her that to that end you will set certain rules with certain consequences, but then acknowledge her responsibility for being the one who ultimately will make these decisions. PS - Alanon might be supportive for you as the parent, even if she is not using any substances, the issues are close enough.
The task of a teenager is to become an adult, when a teen is troubled it is imperative that they learn that the responsibility for their actions is theirs alone. ) to unravel that while the problems may stem from some terrible situations and horrible stuff that other people did to her, that she is the one who must now move forward with her life and find ways to make choices that support her.She needs to prove to you and herself that what you think, is of no concern to her - but it is a ruse - it matters more than she can accept or let on.If you are not condemning (and sounds like you are not) but instead acknowledge the fact that this is ultimately a choice she must make about her own body, she will be more able to hear the things that you want to say about the situation.I find myself in the unsettling position of wanting to say very conventional things like ''An 18 year old boy only has one thing in mind.'' And then I feel bad. Older guys pick on younger, more vulnerable girls because it's harder for them to say no. They mostly spent time together at school, at lunch, or met before school for coffee. I told her that I wanted to meet him before they went ''out'' anywhere. Just make sure she knows that she can call you anytime and you will pick her up. That way she might talk to you about any things she doesn't know how to handle.Talk to your daughter about different things that can happen when she is with him and let her know she can make a choice about what she wants to do. I also tell her if he wants to ''go farther'' than she does, she absolutely has the right to say ''no''.