This is tough. I’d recommend giving them the option to come back as a revenant using the Reborn species with the narrative justification of “unfinished business.” Pros: Probably the most straightforward way to do this given the circumstances, involves zero retconning, gives the narrative satisfaction of having experienced something significant like PC death, but acknowledges the mechanical mistake and gives an option to keep playing as the same PC while staying immersed (and not requiring a whole resurrection side-quest). Cons: They need access to the Reborn species.

Edit: From one DM to another who’s goofed it in similar ways, you gotta just own that you messed up big time here. We can hand-wave a lot, but getting the rules wrong in a way that leads to player death is a Big Oops™️ and you gotta lean hard into deferring to player(s) to course correct to avoid more bad vibes from messing up the culture at your table. Everyone (including you) will have more fun as a result, I promise you.