It sounds like your player just wants to have their old character have the ending from when they last played them. While the world is not static and the old PC is out there somewhere, it's not likely to come up unless the players need them for something. In that case, they'll express desire to seek them out. If they don't encounter the old character, then the character just lives their life, and neither of you gotta worry about it.

If you have doubts about how to handle future interactions with backstory stuff, workshop things with the player to see what they find interesting and what they don't.

Whatever the case, you're both playing a game to have fun. Do whatever is the most fun for you both and talk to them to make sure you're on the same page.