[mEGALOMANIAC]

Uh, so he's combining all the advantages of two armies, with none of either army's weaknesses? He may call you a wuss, but he's cheating to an enormous degree (unless you actually play against this, in which case its partly your fault for allowing it!).

I'm sorry, but this kind of style would get laughed out of my local store in an instant. And we definitely wouldn't play him at home if he's such a poor sport that he calls people names when they dislike his rule breaking.

Just point out that what he's doing is akin to mixing any two armies together. SM and CSM both wear power armor, so let's combine them! Eldar and Dark Eldar share ancestry, so lets make them one army, and thrown Harlequins in while we're at it! Heck, it's no better than mixing Tau and Orks for that matter. If he insists that its ok, mix your two favorite armies together in the same way that he did his and see what happens.