My 17 has had a relationship like this, and she has acted in a very similar way, so I can tell you what we did, what I was advised by everyone in our support system. Every time she leaves against your will, call the police to report her as a runaway. If you think her safety is truly at risk, get ON the police to pursue where you think she may be, use words like "vulnerable person" so they will send someone out to look for her and take more seriously than a teenager who is just leaving mad at parents to chill safely at a friend's house for a night or so. If the police find her, they will return her to your home. Make sure you tell the police you would like their support in issuing a curfew violation, or whatever citation they can. When she returns, call can cancel the missing/runaway report. Eventually, this documented history will bring her into court, and she will face legal consequences of breaking the status offense of being a runaway. She could get ordered into a treatment program, have ankle bracelet monitor, get ordered for random UA's etc. All sorts of interventions that would be more impactful than any kind of threat you as a parent can make. This is when it gets real. Do NOT be afraid to call the police. You have to work WITH them. If she won't listen to you to return when she is out unauthorized. Call or text the boyfriend and tell him he needs to support her returning home asap, and that because your daughter is out without permission you have no choice but to get the police involved. This boy is still a minor for now? Go update his parents on what all you will be doing to get your daughter to fall in line with expectations. Tell them you don't want any trouble for their son, but if he is contributing to her breaking the law, he is going to get caught up in it. And get their support in putting pressure on him that he does NOT want to bring on that mess of trouble. If you do have knowledge that he is directing her to stay out with him, to run away, or giving her any kind of substances, get an order of protection (restraining order) immediately. Good luck!