I ask if he is married as there will also be a family to consider..children possibly ..as I say its a tough one .How does she feel about him now ?
I suspect that there would be a statue of limitations law in place around this. So if you are looking for a legal solution, you'd have to look into that. My concern right now would be has he or is he doing this to others as well. So even if there is no legal recourse in terms of justice for your daughter, it might be a good thing to notify the police anyway.
The rest is up to what your daughter needs. I would support her in whatever she needs to work through this...counselling or just to be able to let it out in talking to you. Take your cues from her, and make sure she feels supported. You might want to consult a professional on your own just to learn about how to talk to and support victims of sexual assault? Just a thought.
This is a tough one so much time later, I think he should be confronted that you know what happened ,he will likely deny it, they always do .But children do not lie about these things ,he was also a minor then so not certain if there would be a legal stance .Its tricky, is he married now?