I was in exactly the same position at 16.
Things to consider, the more you push her to leave the harder she will cling on to him, she is probably in a co-dependent relationship, she will not be able to imagine a life without him. This is going to be very hard for her, but she can do it. The worst thing now is to make an enemy of her as she will then be truly isolated, but you can be forceful in every other way apart from anything directed at her, she needs kid gloves, so still sees you as her safe harbour.
If it were my dd, knowing what I do from my own experience.I would be enlisting support from everywhere, uncovering the abuse nature of their relationship and the danger to your child is the most important thing, to absolutely everyone, you can't do this alone.
Do you have other family members that can step in and talk to her? It is better coming from trusted others, than you - she may well listen
Inform the college, and ask them to get involved - do they offer counselling or support? Can a teachers talk to her about how concerned they are about her work and wellbeing?
Call Childrens services and ask for help. Social services will be able to advise.
Contact the local police station and ask them how to proceed, can they come over and speak to your dd. Is it is a criminal matter - coercive control is now an offence?
Contact the boy's parents and tell the steps you are taking, would they agree to her staying in her own home from now on, and not theirs. I would make it clear you intend to escalate this. No need to blame their son, I think I would say you feel they are too young to be so involed, college work is falling behind etc. This is not about placing blame, but getting as much help as you can to get her out of there.
Inform her friend's parents what has happened to your dd, and ask that they continue communication with her.
I know it is easy to feel embarrassed or shame about this kind of situation, but the more you can expose what is happening the better you be placed to get her back. You do all of these things, whilst constantly telling her you love her, you are worried for her and will be there for her for as long as it takes.
You are going to war to get your child back - but you do it smartly and with minimum fuss towards her mobilising every support network you have. I nearly died (due to the level of violence inflicted on me in the end) as a result of my mother leaving me to it and hoping I would work it out, I did not work it out I was too young to know where to turn - don't run the risk with your dd.