GrapheneOS duress PIN could land a man in prison

To be fair, a duress pin is definitely a good thing to have if you’re being mugged, but using it to deliberately destroy data when it’s been seized by a lawful authority is guaranteed to land you in serious trouble pretty much anywhere. Whether the border agents were, in fact, a lawful authority, and whether they had a lawful reason to seize his phone are very different questions.

The much more sensible approach is to either have a separate phobe for travelling, that has nothing you care about on it, or wipe your phone before you travel, and spend a little time building up an innocuous history on it, without putting anything you care about on it. Anything you absolutely must have access to while you’re away can be stored, encrypted, online, and you can remember the details to retrieve it.

Third option: Refuse to give your phone credentials unless these fascist pigs have a warrant or provide you with a lawyer.

Then they will just deny you entry into the country

He’s a US Citizen. Legally they cannot deny him entry into the country.

Central to these protections is the absolute right of a U.S. citizen to return to their country. This right is unequivocal. No matter where an individual has traveled, the duration of their absence, or their personal or legal background, a U.S. citizen cannot be denied entry into the United States. This principle is supported by both statutory and constitutional law. A landmark Supreme Court case, United States v. Wong Kim Ark (1898), established that citizenship includes the right to reenter the nation. Unlike lawful permanent residents or visa holders, U.S. citizens are exempt from admissibility criteria. Consequently, even if a U.S. citizen lacks proper documentation or is subject to further questioning, the government is obligated to permit their entry once their citizenship is verified.