friend had sex with minor - what should i do?

[pollypower]

My friend has had sex with a minor, he was about 30 and she was 15. They had a secret relationship that lasted about 6 months. Although this happened about 2 years ago, it has just come to light. The circle of people that know - including the girl's parents - are trying to cover it up and keep it quiet.
I don't know what my obligation is here, the girl came to me and told me she doesn't want anything to happen to the guy, she said she loves him and wasn't pressured into sleeping with him. She is scared and upset people will find out and he will get in trouble.
The guy was fully aware of her age - he is also friends with her older brothers.
Trouble is, this is illegal, although the girl says she consented, plus he is also training to be a youth worker. If this 'gets out' I fear the implications for him could be far-reaching.
What do I do?

[pollypower]

thank you to everyone for your advice - some a lot more valid and helpful than others.