I'm not sure I understand that argument. Either the guy was in possession of the methamphetamine or he wasn't. They're not going to find any inside his phone.
BTW, if by unlocking the phone, they mean accessing its content, why can't they just cut off the guy's finger and unlock it with his fingerprint? I'm sure the importance of the case warrants an amputation.
Actually, I don't think that will work. The finger has to be attached to a living person. There are electrical signals that pass through the finger to the screen. That's how capacitive screens work.
A dead finger won't work. It has to be a living finger. . . just as a photo print of a fingerprint doesn't work. . . so chopping a finger off won't work, nor will using your finger from someone's corpse doesn't work. Incidentally, it isn't reading your fingerprint, it's reading the subcutaneous ridges below the finger print ridges. In addition, after not being opened for 48 hours with a either a passcode or a finger, the phone requires it must be opened with a passcode and the "fingerprint" won't work anyway. Also, if the iPhone has been shut down at anytime, it must be re-opened only with the passcode.