It doesn't sound like this person pressed any buttons. They were pressured to provide a PIN or be delayed and further harassed. They obliged, and agents decided to enter it to attempt a warrantless search of the phone.

It's not stated, but probably we can assume the person didn't ask for his phone to be searched - probably he asked NOT for it to be searched, at least based on his multiple requests to talk to his lawyer.

Considering those factors, I'd say border patrol is more responsible for wiping the phone than the person.