You did not indicate in what state your friend is located and in what state the girl is located, and that matters because most criminal law is state law. Because your friend was age 22 when the nude photo of him was taken the photo is not child pornography. It may, however, still be considered “obscene” material. There are federal and state laws that make distribution of obscene or pornographic material a crime. Where the distribution is to adults these laws are very rarely enforced, in part because modern standards of what is considered offensive have changed a great deal since these laws were written and in part because of the constitutional issues that often are raised in these cases. But crimes that make illegal to distribution of obscene/pornographic material to minors are much more likely to be enforced because kids need more protection than adults do. Federal law makes it illegal to knowingly distribute obscene material to anyone under age 16 using any means of interstate or foreign commerce. 18 U.S.C. § 1470. Sending material over the internet will qualify as a means of interstate commerce. Conviction on this offense can get the offender up to 10 years in federal prison. The key, though, is that the government must prove that the defendant know the person he/she was distributing the obscene material to someone under age 16. So if he was prosecuted and could convince the jury that he thought she was 22 he should be able to avoid conviction. But you never know what the jury will believe. Many states also make this sort of thing a crime, too. While I think it is probably unlikely that he’d face prosecution for sending just this one pic when it was connected with a dating web site situation, I can’t guarantee the police or FBI wouldn’t act to at least investigate if an angry dad shows up complaining about a 22 year-old sending nude photos to his 15 year-old daughter. All he can do at this point is wait and see if anything happens. He should not have any more contact with the girl or her father from here on out. Talking with either of them is likely to do him more harm than good.

This situation illustrates why people should not send or receive nude/sexually explicit photos or material to/from people they do not know. People on the internet can lie about anything, including their age. If they do and it turns out they are a minor, you could end up in a world of hurt by exchanging sexually explicit material with them. Had she sent him a nude photo of herself, he would be receiving child pornography, and that can be extremely serious. It’s one thing to exchange this kind of stuff with a girlfriend/boyfriend you have dated for awhile and whose age you have actually verified; it's quite another to do it with someone you’ve just met on the internet. The former isn't likely to cause you legal problems, the latter certainly has a much higher risk of legal issues.