OP at 15 I got involved with a boy who was two years older than me he was lovely for a year then his true colours came out.
He was manipulative, controlling and abusive. I was besotted by him, he has slowly chipped away all my confidence so that I felt no one but him would have me and that I was lucky to have him as he was an amazing catch. So when the verbal and emotional abuse started I felt I deserved it and it was my fault I provoked it.
It went from there to his exerting his control sexually over me and I did things I'm utterly ashamed of today. By the time I was 18 I was still with him but knew it was wrong and deep down I wanted out. He had progressed to thag point to giving me the odd slap but to the outside world he made me out a liar and to be crazy and told people I was stalking him etc.
My parents knew he was emotionally abusive but they didn't really care or act like they cared and they never once told me I was worth more.
My friends begged me to leave him and I would as I "loved" him. Like your daughter I didn't see him much buy the abuse over the phone, text and Internet was there.
At 18 I started spending alot of time with friends at the cinema for meals and I got so much confidence in myself. I found I wasn't waiting for the next contact with him and I decided that I didn't need him or any other man in my life and binned him.
That shocked the life out of him and he begged and pleaded for months and it got worse when I met my boyfriend who is now my DH. He didn't like losing the control but I was free and a completely changed person. Him on the other hand I've heard has remained arrogant and abusive to this day.
Please don't ban her from seeing him it won't work. Encourage her to go out with friends more and do things she enjoys away from him and tell her as much as you can how much she is worth. Do whatever you can to boost her confidence because when she gets confidence in herself she will have the confidence to bin this fucker for good and she won't look back.