united-states

Almost every U.S. jurisdiction (perhaps all of them) recognize the affirmative defense of "duress" to a criminal prosecution. The precise scope of what actions are sufficient to establish that duress is present vary. A credible threat that someone will kill you or your child is generally sufficient in every state that recognizes the defense. There is also a closely related defense known as "choice of evils."

This said, eight years is a very long time and it is probably less likely than not that a finder of fact would conclude that it was impossible to seek to involve authorities in the situation for all of the time in that eight year time period without reasonably risking harm from the threat.

Also, while you call his acts "treasonous", in reality the crime of treason is defined very narrowly in the U.S. Constitution to taking up arms against your country and providing aid and comfort to people who are doing so, and the fact pattern suggested probably does not constitute treason under U.S. law.

Finally, while his circumstances only dubiously are sufficient to assert a defense of duress over the sustained eight year time period, this doesn't mean that authorities couldn't choose to refrain from prosecuting him, or that the couldn't seek only lesser charges with mitigating circumstances conceded in sentencing, in an exercise of prosecutorial discretion. Often this exercise of prosecutorial discretion would be conditioned upon cooperation in the case brought against other defendants in related cases.

In an exceptional case, the pardon power could be invoked for relieve him from criminal liability as has been done in some past insurrections in U.S. history like the Whiskey Rebellion and the U.S. Civil War. Even treason is a pardonable offense at the federal level.

england

While I am not an expert on the exact details of English law, in general, it is harder to claim the defenses of "duress" and "choice of evils" under English criminal law than it is under the criminal laws of most U.S. jurisdictions.

united-states

Almost every U.S. jurisdiction (perhaps all of them) recognize the affirmative defense of "duress" to a criminal prosecution. The precise scope of what actions are sufficient to establish that duress is present vary. A credible threat that someone will kill you or your child is generally sufficient in every state that recognizes the defense. There is also a closely related defense known as "choice of evils."

This said, eight years is a very long time and usually the defense of duress is available only if the threat was imminent or immediate. So, it is probably less likely than not that a finder of fact would conclude that it was impossible to seek to involve authorities in the situation, for all of the time in that eight year time period, without reasonably risking the threatened harm.

Also, while you call his acts "treasonous", in reality the crime of treason is defined very narrowly in the U.S. Constitution to taking up arms against your country and providing aid and comfort to people who are doing so, and the fact pattern suggested probably does not constitute treason under U.S. law.

Finally, while his circumstances only dubiously are sufficient to assert a defense of duress over the sustained eight year time period, this doesn't mean that authorities couldn't choose to refrain from prosecuting him, or that the couldn't seek only lesser charges with mitigating circumstances conceded in sentencing, in an exercise of prosecutorial discretion. Often this exercise of prosecutorial discretion would be conditioned upon cooperation in the case brought against other defendants in related cases.

In an exceptional case, the pardon power could be invoked for relieve him from criminal liability as has been done in some past insurrections in U.S. history like the Whiskey Rebellion and the U.S. Civil War. Even treason is a pardonable offense at the federal level. At the federal level in the United States, the President can issue a pardon even if someone has not yet been convicted of the crime in question.

england

While I am not an expert on the exact details of English law, in general, it is harder to claim the defenses of "duress" and "choice of evils" under English criminal law than it is under the criminal laws of most U.S. jurisdictions. See here.