It doesn't actually matter, unless he's KNOWS he's downloaded something illegal and KNOWS how to find it on his computer, or has opened it already and not delet

It doesn't actually matter, unless he's KNOWS he's downloaded something illegal and KNOWS how to find it on his computer, or has opened it already and not deleted it. Basically all possession laws (really all laws in general) require some sort of knowledge and intent. Just like you can't be convicted for possession of drugs if they were stuck to the bottom of your shoe when you came out of a club (don't actually try this), you can't be convicted with possession of illegal digital material if you weren't aware you were in possession of it. I did digital forensic work at an old job and one of the cases was someone who liked to indiscriminately download huge amounts of porn off dodgy p2p applications. We found gigabytes of indecent images of children, but no searches in the p2p application for commonly used IIoC terms, and no evidence that he'd either opened the folders containing the IIoC or actually viewed any of it. Police gave his hard drives back (after wiping them ofc) and let him go. I wouldn't be concerned about his 4chan backups, any lawyer will be able to get him off the hook for anything illegal in there provided he hasn't actually opened it.