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.
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.
Past efforts to monitor a dating situation have failed, either because of being lied to or because of being let down by a past boyfriend's dad who promised to supervise if the two spent time at his house when he WAS there and did not. Knowing I cannot control what she does, I want to set age- appropriate parameters and attempt to hold her to them. Also, these parameters cannot be based on expecting truthfulness or abstinence.
They have to be based on something else, giving her reasonable guidelines within which to learn to take responsibility for herself.
I'm generally very liberal but not when it comes to early dating.
Our daughter has a nice social life and is not suffering by not dating solo.