This colleague that I mention in my scenario (How to handle being assigned a colleague's work when the colleague does not complete it) have decided to leave the company because he/she have manage to find greener grass out there.
So, would it be right that I not complete the task that was re-assigned by my boss and persuade my boss to agree with my action. (so that the colleague will not use the work that is performed by you and list inside his / her resume.)