she turned 17 in aug.
This is a criminal issue. If the defendant is more than 3 years older than a minor they can charge it as a felony.
This is just my opinion and not a comprehensive answer. You assume the risk because this answer may not apply to your situation depending on the facts.
Divorce / Separation Lawyer
Yes, sounds like a felony to me.Unless they were married. Other states differ, this concerns Calif only. Here's a link not intended as legal advice but just FAQ
The answers on this discussion board are general in nature and NOT intended as legal advice. Responding to questions does not constitute an attorney-client relationship. Always see a lawyer about your individual situation.
Child Custody Lawyer
Everything the above attorneys say is correct. But they failed to say that authorities usuallly do NOT file charges in these cases, assuming that the sex was consensual and not rape, because their child needs financial support (especially if mom is 17), and sending dad to jail or prison will effectively prevent father from paying child support.
Dad should retain an attorney (me), before hearings begin, to try to feel what the police, and DA feel about this case, and try to smooth over things to reduce the chance of criminal charge being filed against father
I am a Sacramento attorney, representing clients in court in Sacramento and in surrounding counties. Or, if you cannot afford full representation then, for much less, I can instead write you the forms and declaration you need, and help you serve them, and you can go to court by yourself. A well written declaration, by an attorney,supported by proper evidence, will greatly increase your chances of sucess in court. I take credit cards.