Title 18 of the US Code (18 USC) is the entire federal criminal code.
18 USC Sec. 2382, Misprision of treason. Unfortunately the max sentence is 7 years.
Whoever, owing allegiance to theĀ United StatesĀ 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 theĀ United States, or to the governor or to some judge or justice of a particularĀ State,Ā is guilty of misprision of treason and shall be fined under this title or imprisoned not more than seven years, or both.