Comment by grapheneos

A court has not yet determined whether the use of the duress PIN/password was legal. There's definitely no consensus among legal experts of it being illegal as you're portraying it. The US has strong legal protections against self-incrimination and unreasonable searches despite erosion of how much people's rights are respected.

A factory reset done in anticipation of a search is not as different from using a duress feature as you believe it is. Forensics software would have clearly identified the device was recently factory reset. It would provide another defense argument by arguing it was wiped for another reason, but whether that would be believed by a court is unknown. It would make a difference if there was a good argument about why it was done, but it isn't necessary for this to have been done instead for wiping the device to have been legal.

Once he was in the situation already, the best move was very likely refusing to provide the PIN/password indefinitely and only talking to them to demand access to lawyer. There are strong protections against data extraction and it's highly unlikely they would have been able to get the data from it. Refusing to provide a PIN/password is protected under the 5th amendment in the US and these rights do exist at the border. They can turn away a non-citizen but they can't refuse entry to an American citizen because they won't provide a PIN/password. They could waste a lot of his time but he'd get access to a lawyer and would get released. They could make a court case over demanding the PIN/password and they'd nearly certainly lose. He'd likely spend months or even years without getting back his phone of course.

If they had a video recording of him entering the PIN/password from somewhere, they could have used that to get the data. By using the duress PIN/password, he prevented it. It was probably not necessary to keep the data safe, but that's unknown.

With only a tiny bit of preparation time, rebooting or powering off the device would have gotten it into Before First Unlock state without the locked device auto-reboot timer needing to complete. In Before First Unlock state, a decent random 6 digit PIN is enough for the data stored protected with it to be highly secure without an extremely sophisticated secure element exploit. If the device had a strong passphrase, then no level of sophisticated exploits would recover that data.