[Barnstormer]

It's called Misprision of Treason and it's covered by 18 USC 2382. If you read the statute you'll see that that he has an affirmative obligation to report such knowledge to Federal officials. It's punishable by up to six years in prision and a fine.

[John Thornton]

Sec. 2382. - Misprision of treason

Whoever, owing allegiance to the United States and having knowledge of the commission of any treason against them, conceals and does not, as soon as may be, disclose and make known the same to the President or to some judge of the United States, or to the governor or to some judge or justice of a particular State, is guilty of misprision of treason and shall be fined under this title or imprisoned not more than seven years, or both

[230FMJ]

I assume that's for a civilian. I wonder if the offense is made more serious by the fact that he still held an officers' commission in the US Navy?