Forum:Clarify Changing Destiny

Had an argument with my bother over this. He argues that while I am picking the effect it applies from his perspective since he is the one that played it, meaning he gains life or I take damage. I say it's the other way around. Is there an official ruling on how this effect works? I've seen the semi-official ruling, but I want to know if there is a 100% official ruling on how it works.