I really can't think of who it could be. I can't agree to suggestions of Verin or Forsaken, because I find it unlikely they might be able to adequately impersonate an Aiel what with their complete isolation until a few years ago. And I'm not convinced it was done in a dream either, while Aviendha might not have realised she was dreaming at the time, surely she would have realised it upon waking up. But it is true that the scene exemplifies some of the peculiarities of dreams.