Then it's normal for people to have encrypted stuff on their drives that they can't decrypt. And thus a "reasonable man" could not be expected to be able to decrypt such stuff even if he cooperated fully. They could be using full disk crypto with an encrypted container file that they can't decrypt. They can decrypt the first but not the second (or maybe they can - it becomes harder to tell
But once a popular OS has stuff like this by default, it's much easier for the defence to argue that you can't do it.
Of course in this case - the guy has been supplying wrong passwords, so unless you can show it was out of desperation and/or due to duress, he'd still be in trouble.