This is a tricky one.
A few years ago, I had a contract with Microsoft to check for child pornography on a selection of free websites. These were for hobbyists, so I saw more photos of the inside of Vauxhall Nova engines than you would believe!
Thanks to our access codes, were were able to access a site without the owner knowing - they would simply shut it and open another. If we found anything, we'd report it to MS, who passed it on to the police - we found out afterwards that this was Operation Ore.
I learned during that time that sharers are incredibly devious, and often like to 'hide in plain sight' - operating an apparently harmless front page, with the images inside the site if you clicked on. I recall one site being about nuts - the edible kind as an opening page, and if you clicked in - he presto, Level Five images.
So i can understand how someone could download a file in all innocence, but i think this man was seriously naiive if he thought he could take it to the police and not be implicated in any way.
I know we all have hindsight, but I would have deleted the image, bought a new PC and taken a hammer to the original hard drive.
After all - the police only have his word for how he obtained these images - and hiding in plain sight is a ruse to distribute these images, as I know from experience.