The charge isn't for possessing a #GrapheneOS device, it's for destroying evidence. I'm not convinced it'll hold; it doesn't sound like a warrant was issued at the time when authorities wiped his phone by way of the duress pin.
Still, It is probably better to simply leverage a short Auto Reboot window (<=2hr) in order to force reboot the device, making it's contents encrypted at rest.
Then, simply assert that you are happy to cooperate through your attorney, as you are not a legal expert, you must obviously use your right to remain silent, as everybody with rights should.
