I went out with a 20 year old when I was 15. We didn’t have sex, he was an immature 20 year old and I was a mature 15 year old.
He was very much of the “no sex until marriage” persuasion but I know that his mum was worried about him being with someone so young.
My parents never actually met him because I went to boarding school, but my mum had a talk with me about sex and contraception, and her view was very much that I needed to be sure I was safe and protected.
Banning me from seeing him would never have achieved anything.
We broke up after about a year because, well, I grew up and he didn’t, so the relationship essentially just ran its course.
It used to be a lot more common for younger girls to go out with older boys. Statistically boys are less mature than girls which is why so many 15/16 year olds don’t go out with boys their own age.
You need to talk to her, have an honest conversation about A, the fact she’s under age, B, the fact she needs to use protection, i.e. condoms and the pill if need be, and C, that you will always be there to talk to, no matter wat.
Wading in with talk of abuse etc is only going to drive you further apart and potentially bring them closer together.
And she’s friends with his sister. To keep them apart you would have to ban this friendship also, and that’s not realistic.