You have to remember if the person is thoroughly incompetent, none of this will work, but imagine if the person is actually pretty smart and good enough to get by, then make them personable and friendly ... and throw in a manager who doesn't really know shit about what you do ... then its a whole new ball game. In fact someone like this could wind up getting promoted over you into a "architect" role or some quasi-dev manager role (bosses who don't really know what developers do, love over communicators). I've seen it happen.
Often times, they can gather a mob (depending on their social skills) and push out other engineers they don't like, or completely comandeer the engineering organization into ill-fated directions.
What I'd like to hear from the OP is ways to counteract this kind of behavior, I suspect there might not be a way to do it if you're a peer, only if you're that person's boss.