I cannot believe I purchased this game and an expansion, then played for three nights and now I cannot play with my friend anymore. We arrived at homestead, I chose minion armor, he chose fire damage. Now we cannot progress through the game together. This is baffling to me. Can I please have my money back?
You can progress through the game together just fine… just now you will kill Kymon’s chosen and your friend will kill your necromancer friends. There aren’t that many areas that only consist of those enemy types. If you want to do your faction quest you will have to play solo, use a cheat tool to change your faction, or choose the same faction next playthrough (elite or with new characters.)
What do you mean by “cannot progress through the game together”? Death’s Vigil and Kymon’s quest lines are entirely optional and separate from the main quest line.
The only issues I’ve ever heard of (and briefly experienced) with your situation is the player who sided with Death’s Vigil was allied with the Necromancer NPCs you encounter and hostile with Kymon’s forces and vice versa and neither player can join in on the other’s associated quest lines (understandably as lore-wise, both factions are rivals/hate each other).
If it really is a deal breaker to you now, let me just say that Kymon’s and Death’s Vigil’s forces only appear from Blood Grove to around Fort Ikon, do not make any appearances in the expansion content and finally, every difficulty gives you the option to make faction choices again so you and your friend can side with the same one in Elite/Ultimate.

No, really, I’m horribly confused here. How does you making a choice in-game that you have decided you regret equal “REFUND NAO”?
It all but spells out for you that you will be hostile to Kymon’s Chosen when you choose ODV and vice versa, so it’s not like this was an ambush decision or anything.
I laughed.