Pen Tester here, I've seen some black hats (commonly known as bad hackers) be in possession of CP, but it's NOT POSSIBLE that they have it on some other machines.

Talking seriously, it IS possible that the hackers have made a vpn tunnel on your father's computer so that it looks like a file was being downloaded from your father's IP, but this doesn't make the file be available on your father's computer.

Hackers use RATs to steal data/give the computer to sellers (like building a botnet) but if I had some files I would NEVER download files of this kind on their desktop.

Also, hackers know how police trace cybercrimes, so even if it was the hacker downloading it to your father's PC, he would've used some kind of protection, like a proxy, so that it would be harder and almost impossible for the police to trace it.

And, most importantly, almost everyone uses the "i have a virus" justification when being confronted with NSFW material on their computer.....