Well your friend is an idiot :P No class or race is really difficult to level but that being said if you have a non self healing class such as rogue or warrior undead would probally be the easiest and with a self healing class humans would be the easiest as they can prevent long stuns so they...
I am starting up a power leveling service currently with combat, minning and agility and offering my services to the first 10 people free, because this is a free service at the moment i'm only offering people up to 15 levels or 300k xp worth of leveling (which ever is less) and you must supply...