the Founding Fathers are spinning in their graves
Taking the Fifth (invoking the Fifth Amendment (against self-incrimination)) is as good as an admission of guilt these days - after all, if you didn't have anything to hide, you wouldn't object to them stomping your Fourth Amendment rights (to privacy) with their hobnail boots.
Also, what browser saves file hashes?
Or, if they were torrented, then the names and locations (on the external drives) would be known, in addition to hashes, at which point, that would be sufficient to say that he had said images in his possession at some point (as well as having distributed them). At which point they wouldn't be demanding his keys and the judge wouldn't be making up laws.
If his lawyer is any good, he'll get off.