I have heard from many that even if you have the minimum specifications for World of Warcraft, you have to turn down all the graphical options, and it looks pretty bad. My brothers tried installing World of Warcraft on my iMac G3 400 Mhz, and the game ran so slow that their character wouldn't even move. And that was even with all the graphical options turned to their minimum. So, your friend's computer may run better than that, but most likely still sluggish enough that keystrokes may not register quickly, making the game frustrating.
I found the specs for the last iMac G3 ever released
here. You will notice it is under the minimum requirements in many areas.
If you want the graphics to be amazing, you should go for a Mac Pro. But, I have an iMac G5 with 2 GB RAM, 2.0 GHz G5, and an ATI Radeon 9600 with 128 MB DDR and World of Warcraft runs nicely. I haven't played, but my brothers have, and they both have no complaints about how my iMac runs it. They are even able to multitask (browse the web, run Ventrillo/TeamSpeak) when they are playing on my iMac G5.