Nothing much
See here on CPS guidance page:
http://www.cps.gov.uk/legal/section7/chapter_f .html
Code for Crown Prosecutors - Considerations
The public interest requires the prosecution of an offence of unlawful sexual intercourse with a girl under 13 unless exceptional circumstances exist.
You may exercise considerable discretion in relation to offences under Section 6 of the Act.
The age of the defendant will be highly relevant. Even if the defendant is over 24, a prosecution may not be in the public interest if he had reasonable cause for believing that the girl was over 16.
The following factors will also be relevant:
the relevant ages of the parties;
the emotional maturity of the girl and whether she entered into a sexual relationship willingly;
the relationship between the parties and whether there was an existence of a duty of care or breach of trust <refer to Sexual Offences - Abuse of Trust, elsewhere in this guidance>.
In summary, a man who is considerably older that the girl is likely to be prosecuted, especially if he owed her a duty of care; whereas it may not be necessary to prosecute a young man with whom the girl has been having a consensual relationship.
So where the ages are relatively close as here it's unlikely to result in a prosecution had he been late 20s or older it might be different.
Do I take it that the girl is your daughter?
I don't think we know enough about the circumstances or personalities to give you advice. Some 15 year old girls are quite mature enough to make decisions like this for themselves whatever their parents might think and others are not.
I'd bear in mind that if you go in heavy handed you may well stregthen the r