It is possible if you have money to spend on this game. It is not possible for someone to do this without spending money.
It is likely they could have just leveled up to 30 got an outrageous power level by purchasing gems and then went to town in the Cretan Labyrinth. Not to mention that if you boost your power level really high you can get through a slew of quests that offer tons of exp past level 30.

So it is very much possible to hit level 60 in 3 days. Must be a dedicated player!