Go to the police and report what you know. The police will probably investigate and if he doesn't admit or confess, at least he'll be quite frightened about his sex with a minor, might make him think twice about this relationship even if he doesn't get prosecuted.
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Proving statutory rape is usually accomplished by having the "victim" state she had had sex with someone. Another way to prove it is to have the perpetrator admit to the sex. Another way to prove it is to have a 3rd person who saw the two having sex, or who overheard one of the participants talking about having sex with the other, talk to the authorities. Without either participant or a 3rd person who saw it or overheard an admission coming forward, it is hard to prove.
You can call the police. They will have someone meet with your daughter or even the male. Be sure you are prepared for the consequences of your actions. This can become quite ugly for your family.