i don't understand why it's locked as i wanted to answer your questions which is, "just got in a game with somebody with the gamertag blah, blah, blah. he had every perk when i watched the kill cam and was invisible on the map prison break..."
are you sure he wasn't using the specialist package? where you can build more and more perks as you kills mount up... not sure about the invisible thing, i'm thinking you just missed them.
I cant say you are worng or right in this case but in Black Ops (I think) people were complaining that someone else had a hack where a yellow triangle would show up when you stunned someone. When it turned out it was a perk in the game and some people assumed it was a hack.
As for the fact about it being allowed there is a post about 5 posts below yours with almost the same title. That has the answer