>In the abstract sense, it is wrong to invade privacy.
You have no real expectation of privacy when using company owned equipment. This was almost certainly spelled out to the employee in question in the acceptable use policy he agreed to upon being hired. Companies have to operate this way so they can investigate computers if compelled to by court or law, and so they can recover important information off computers when the user exits the company.
If he was using a BYOD computer I'd have a different opinion on the matter.