There are a lot of standards in gaming. Or, at least gaming faux pas. When playing an RPG, you don't roll your friends dice as your own. Players of X-Wing know the saying "Fly Casual".
Now, there are several cards that say re-roll 1 or more dice, and that target may be yours or your opponents. The question is, do I, as the owner of the card being played, re-roll my opponents dice if that is the target of the card being played? What will be the standard practice in Destiny?