Forum:Dark Fusion ruling help

Hello there, I am new here and I need a bit of clarification on the ruling of Dark Fusion. My friend and I were dueling and he Fusion Summoned his Elemental Hero - Golem and attacked with it. I retaliated by activating Mirror Force. Clearly, I thought that his Golem should have been destroyed. Him on the other hand, stuck to his guns that the fact that Dark Fusion says "cannot be targeted by your opponents spells, trap or monster effects", his Golem was not destroyed. He did however destroy all his other monsters that he had except the Golem. I know in my gut that the ruling I had in mind was right. The Golem should have been destroyed since Mirror Force doesn't target. I also know effects such as Zero Gravity would work on it the turned that it was special summoned because it, too, doesn't target. I need some clarification on this, please. Thank you!