Cream of tartar is used to stabilize egg whites and acts as a leavening agent (helps cakes rise). According to Wikihow you can substitute white vinegar, 1:1 ratio for the cream of tartar. So for this recipe, you would add 2 teaspoons of white vinegar to the egg whites while they are beaten. I would also add just a bit more sugar substitute, perhaps a tablespoon, to counteract the bitterness of the vinegar.