Another explanation would be something like what happened to Alphonse Elric from Fullmetal Alchemist.
Your Pal winds up sharing control of the Warforged's body for a limited time, something the Warforged is willing to do...but there will be consequences. Sometimes, the body has control problems (negative dex mods), or they struggle over who is in command (negative initiative mods).
i disagree with the sentiment of retconning anyone who cant take the death of his character needs a new game. my friend made a druid. first battle and his animal ate diseased meat and died. then another pc killed the guy. we really dont like his play style. so we made him role a new guy. he wanted bard but we made him roll an elf ranger. i allow some retconning. like taking back a spell that is pointless. sometimes if i get mad and kill someone by accident ill undo it but if they cant take the fight they need a new character or you need to give weaker opponants. but to your porblem if he absolutely wants to keep that character. you could have bahamut revive him. just a thought to make it harder he is stripped of being dragonborneuntil he goes and redoes the ritual maybe. just an idea
okay first i said i didnt like ret conning especially a massive thing. but like
npc: says something important.
pc: i cast tongues on myself so I can understand him
dm: dude hes speaking common...
pc: oh then i dont do that.
as for making him roll ranger. that wasnt as a dm that wthat is what i was talking about. as a friend that saw his playstyle. and he is a beginner. who was trying to play classes that he shouldnt have been. he loves his ranger btw thanked all of us for making him do it.
as for god resurrecting he is the god of dragonborn. hello so theres ample reason add to that theres not a good way for the pc's to do it right now.
it would be acceptable. im sorry if you dont like my playstyle i like to let my players have some fun instead of worrying about very decision
i do have a question though how is there an evil dragonborn. their purpose is to fight evil draginkind so how did one go evil?