Legal Question in Criminal Law in California

If the police are involved, the most important thing is they he say nothing. NOTHING.

Yes, he could get in trouble. In a case of unlawful sexual intercourse, an honest and good faith belief that the minor was over 18 is a defense. While technically not extended to all sexual acts with under age parties, the analogy is there.

He needs to save every single message between the two of them that will demonstrate the deceit of the minor by misleading him in the true age. He may want to consult face to face with a very good local criminal defense attorney to discuss things, should this move to a criminal case.

I agree with Mr. Dane, and I want to elaborate on something he said.

A good-faith belief that the minor was over 18 is what's called an affirmative defense. That means the defendant is the one who must prove it if the case goes to trial. The prosecutor only has to prove the elements of the crime(s) at issue, and must do so beyond a reasonable doubt. Knowledge that the minor was under 18 isn't an element of the crime, so the prosecutor doesn't have to prove it. The defendant can try to prove he thought the minor was an adult, and must do so by a preponderance of the evidence.

This may not sound like it matters much, but it does. If knowing the minor's age was part of the offense, then the jury would have to acquit if it had even a reasonable doubt about what your friend knew. Because it's a defense, they can only acquit if they believe there's more than a 50% chance he didn't know.

Also, in order to establish what he believed, your friend would almost certainly have to testify. Criminal defendants can't be forced to testify in order to rebut the charges against them. But presenting a defense is different from rebutting the charges. It's sometimes possible to present a defense without the defendant's testimony. This very likely isn't one of those times.

Of course, all of this assumes your friend is prosecuted. That might never happen. But if it seems likely to happen, then your friend will need a competent defense lawyer right away. In the meantime, as Mr. Dane said, it's important that your friend not discuss the situation with anyone except lawyers he's consulting with and members of their staffs.