Griefing, in and of itself, is a bit of a nebulous term and often doesn’t consist of activities that are considered a violation of policy. I think “harassment” would be a more accurate term.
I can’t really say there are no situations in which in which we may considered PvP activities harassment but they tend to be rare. Mind Controlling a PvP flagged character from the opposite faction was possible in Vanilla, and I don’t recall any kind of diminishing returns, which would otherwise help mitigate this happening for long periods of time. I’m sorry, but as far as I know that is simply part of Classic.
There is a PvP solution, in that your friend can seek assistance from other players on their side who can kill the Horde controlling him.
It takes about 15 minutes for the game to kick the character out.
Sorry, Sephrinx.