You can access anything a computer has done, anytime, so if its that serious, send his computer off to be investigated.
No matter how many times you delete files, clear things, EVERYTHING you look at has a record kept on your hard drive, and you need a special program to wipe this section (no a format and re-install wont hide it
)
So get a forensics team to look at the PC, theres your answer if it was viewed or not.
Can't help who may of actually done it though.
But the important thing, was the said porno any good