Assuming he's using a method which is against the ToS then it involves using a third party application that either simply enables follow in Battleground / Arena or simulates `/follow` like a bot would find a plant for example. In both cases, it directly modifies the memory of the WoW process which, long term, always ends up in a ban.

The part that surprises me is that you guys are saying that he's been doing that for a while. He must have been reported a lot of times. But still, he's still doing it. Maybe he's using a way that is not against the ToS?