If it was my little girl I would be heartbroken, I'd try not to interfere as it will just push them together, but I'd do my damnedest to make sure she also carried on being a child and that I was very available to her. I hope to hell I'm never in this position as she gets older.
I base this off my own experiences of being in a relationship with an older guy at a similar age. He wasn't abusive, and we loved each other very much. But in hindsight it was terrible, one of the biggest regrets of my life and I harbour a huge resentment against him for carrying it on.
The thing is, however mature you are at 16, you are still changing so much. It's your last chance to be carefree, to not have adult responsibilities, whilst being old enough to experiment with adult experiences.
By being in a serious relationship with an actual adult who is going to have adult expectations of what that entails (I'm talking in terms of commitment, emotional availability, mutual support and respect here, sex isn't relevant) it forces you to grow up too fast. It forces you to always be sensible and responsible. It just isn't fair.
With the hindsight of being an adult now (I'm older than you), I feel like I was robbed of my last moments of childhood by this man. I feel like I never got a chance to experience those simple teenage freedoms of carefree tomfoolery, because I was so wrapped up in an adult relationship that needed my time and commitment and maturity in a way that was just incompatible with being a normal 16 year old.
I massively resent him for it now. As time goes on and I realise more and more what I missed it makes me angrier and angrier. He was an adult, he should have known what he was taking from me, he should have loved me enough to set me free. He had his chance to be a dumb 16 year old, but he took that from me.
He wasn't abusive in the traditional sense, it's just in the nature of an adult relationship to have certain expectations that are incompatible with late childhood.
No matter how aware of it you are and try to compensate by encouraging her to go be foolish with her friends you are going to think the shit they do is dumb and silly, and she will know that and it will steal the fun from it for her, and so she won't do it or won't be able to really enjoy it, and she will grow up too fast, pushed into an adult relationship with adult expectations on her heart, mind and behaviour that your mid teens are just not a place for.
It's not abuse in the traditional sense, but as the adult who had your chance at that time, you should have the maturity to see what you are taking from her. If you know you are stopping her from living a normal life, then, well, it kind of is abuse. The older I get the more I realise what was taken from me, the more I hate the man involved. He should have loved me enough to set me free. He was the adult, he must have known. It was cruel.
So that is why I would fucking hate you. That is why people think it is wrong. And why I pray to god my daughter has the actual maturity and wisdom to not end up with a much older guy during that critical time in her development. The reason I couldn't see how wrong it was whilst I was involved in it is because whilst I was "mature" at that age, I just wasn't an adult with the life experience to see the bigger picture, because I wasn't actually mature or wise, I was a child still developing, and he was an adult and he should have known. And despite us loving each other, and it ending amicably, and the fact that we stayed together for a good few years, looking back now he is the only ex I hate.
He should have known better, I was too young, too young to be in that relationship and yet too young to know any better. He should have known. He must have known. I guess he just didn't care enough, and that's why I hate him - not only did he rob me of my childhood he didn't even really love me, or he would have let me go.
Look back at your own childhood, you late high school years - remember what stupid things you and your friends were doing for larks, what friendships were like, how much you still relied on your parents, how intense your relationship's with your families still were, how much siblings could annoy you, the dumb things that made you laugh, the silly things that stressed you out compared to adult responsibilities and your adult perspective. And tell me you and your friends would have been ready for an actual adult relationship with actual adult responsibilities and needing to emotionally be there and be equal with a fully grown adult.
Set her free. Both be heartbroken for a few months. If you still love her don't contact her, let her grow up and in 5+ years get back in touch when she is actually grown up enough to be your equal. If you love her enough then be the adult, let her go and let her grow.