Just in case weight of numbers has any influence on your discussion, I completely agree with the interpretations above. To my mind, a spell can only be dispelled if it is successful in the first place (being cast successfully is not the same thing as having things actually happen to the enemy). In other words, the success of casting the spell is the wizards job, having a game effect is the spell's job. The dispeller gets in the middle of this process but doesn't effect the intial casting.
