Are you both running WoW from 7200 RPM hard disk drives, WoW constantly writes to HDD as well as reads, so if he is running SSD or Faster HDD, that will make a difference.
Yeah, we're both running 500gb 7200RPM drives, however his is a Western Digital drive and mine's a Hitachi drive. Not sure what difference that makes but that's the only difference I can think of. Does his PC being brand new have anything to do with it? I think it would maybe help him a little, but surely it doesn't justify the 20fps gap between us?