Well, there's the problem of getting the human to be sufficiently well-meaning, as opposed to using Earth as The Sims 2100 before moving on to bigger and better galaxies. But if Friendliness is a coherent concept to begin with, why wouldn't the well-meaning superhuman figure it out after spending some time thinking about it?

Edit: What I'm saying is that if the candidate Friendly AI is actually a superhuman, then we don't have to worry about Step 1 of friendliness: explaining the problem. Step 2 is convincing the superhuman to care about the problem, and I don't know how likely that is. And finally Step 3 is figuring out the solution, and assuming the human is sufficiently super that wouldn't be difficult (all this requires is intelligence, which is what we're giving the human to begin with).