Wow, that sounds frightening and confusing to be dealing with. I am going to be the voice of dissent however. Your daughter needs not be restricted, nor the boy prosecuted, imo; she needs to be empowered to believe that she can say "no" about her body. The choice to continue a relationship with someone who *may have* violated her consent, did not start with this guy. I believe that a child who has been taught to ignore her own right to say "no" about her body, through "doing what you are told", for instance, learns that others have the right to "make her" do things that she doesn't want to do.
If you can find a way to *listen* to her needs, without judging them as ridiculous that she is emotionally attached to the guy; hopefully, she will be able to see you as a partner. From my own experience, I felt that my parents "did not understand" the intensity of my feelings for my teenage love. He, however, did respect my right to consent and we have been together since 1979. I listened to myself, in spite of being restricted from seeing him. I did see him because I loved him. To me, the principle of having autonomy over one's own body and decisions does not only involve young men. It starts at home.
The dissidence that she is expressing about whether she "consented", or welcomes the sexual aspect of the relationship, is not different than ignoring her own feelings because others think that they 'know what is best for you'. For instance, forbidding any contact with him, denies what she is telling you she feels, imo. Instead, I hope that you can open up conversation by listening to what she does love about him, and what she doesn't love about him, according to her own values. Empower her to listen to herSelf, rather than to you or to him. The concern with teaching our children to comply to authority, is that they do, without deference to their own judgement. Help her to listen to her own inner voice, so that it becomes stronger, not overpowered by yours, or his. And then she will be able to know *how* to listen to that inner voice when the next man or woman tries to overpower her own judgement. She will become confident in stating that her consent was or wasn't violated.
There is no standing for a claim of "rape" when she is consenting, imo. I can only imagine how livid I would be if my child told me she had been sexually violated. Perhaps she was and doesn't even believe it. I agree that a neutral party to provide a sounding board could be beneficial, if she consents. I highly recommend the book "How to Talk So Kids Will Listen, How To Listen So Kids Will Talk".
http://www.amazon.de/How-Talk-Kids-W.../dp/0380811960
It has communication tools of validation and empathizing that open up conversation.
Respectfully, Pat