I just found this now, am not sure if it has been posted before but i have not seen any mention of it.
My friend recently hit level 60 and wanted to kill hell diablo, as he was weak and diablos health went up by 100% it made it incredibly hard for my nuke build demon hunter to carry him through the entire fight, so what i decided i would try is to kill diablo solo in my own game, and then invite him as diablo is on the floor screaming before the quest actually becomes "complete" theres a good 5 - 10 second period when he has already been killed where you can still attack him before the quest becomes complete, so i invited my friend as he hit the floor, the quest completed and he got it from town, then he tp'd to me to hand in the next part, and then had act 1 inferno unlocked without actually having to kill diablo as he was undergeared.
I hope this is helpful to anyone out there and if you need to ask a question just ask