He accessed the images remotely, so was a work computer but from outside of work.
He was remotely controlling the pc at work from his pc at home?
Because if so, he would have had to disable functions on the work pc & gathered info inc IP addresses to gain access to the pc.
I had to have a friend remotely access my pc, from his. I am in Australia, he UK. I had to go through several steps (changing permissions etc) before he was able to log in. I also had to give him various details to be able to do so.
So, in order to be able to download these images, he would have had to jump through several hoops to do so.
Therefore, I would be very surprised if it was accidental, and coupled with the other info on this thread, especially how he was found guilty etc, points to him being a big fat liar, who has sucked your sister in. So it would have been very premeditated.
Even if it were a work laptop, it still shows intent was there, as otherwise he would have used his own pc, no?