Is VSYNC on?
Otherwise:
games and graphics card drivers are better optimized for Windows as well as coupled with Direct X, which OS X lacks.
the 5870 should be able to tackle WoW at above 60FPS on a 1920x1200 screen, I'm sure if you install Windows and run WoW you will notice this.
If your friend is running on a smaller resolution than you, that also will make a difference. For instance if you are trying to run WoW at 2560x1600 and your friend is running it at 1280x800 then he will have the benefit of needing to process less thus a higher fps.
We are lacking alot of info like:
Yours and His Resolution
Your OS X version, and if graphics updates have been applied
Yours and His Graphical Settings in game
More specs like: RAM and Friend's Processor
So its hard to make a final conclusion.