What's he going to say? "I know I deserve an identical grade as my friend because we did all of our work together."

Their work isn't literally identical - just almost certainly the result of a level of collaboration that the school would probably regard as cheating. Ironically, the kid who's beefing with his grade was a stronger writer and made his points more clearly on the work he did turn in and on the final exam. He'd have been fine if he'd gotten me his work.

I don't have any direct evidence of them cheating, but if he pushes too hard on that line of argumentation then he's at a real risk of admitting to it.