What makes the contents of his phone evidence? They stopped him for further search because they were investigating him. But crucially, he was not aware he was under investigation and was not told he was. They didn't follow the protocols that would have let him know he was being detained while they investigated a crime. As far as he knew he'd been chosen randomly for a secondary screening.

Then, instead of asking him questions relevant to what he was actually being investigated for, they started asking him about CSAM and demanding he unlock his phone. You are not required and they cannot compell you (in the USA) to unlock your device using a pin or password (but they can do so if you use biometeric unlock like face ID or fingerprint).

So in reality, what happened is they requested a pin and he didn't provide the correct pin (which is his right), and the pin he did provide wiped the phone when the agent entered it. He didn't destroy anything. They did.