TLDR: How best to have a one session guest player play a previous PC as a "baddie"?
I hope none of my players read this subreddit haha
I had a player who played in my IRL ToA campaign who had to move to different state. Everyone loved this player and her character, so instead of just dropping her character (Human Paladin) into an abyss ("you wake up and have no memory of any character by that name that was ever with you"), I had the character get captured and the party goes to rescue her. They end up discovering that the character was captured by the Grung and I played the character as an NPC now. But, because she has an anthropologist background, she says that she doesn't want to rejoin the party bc she's decided to study the grung instead. I really loved the character and hoped that I've have the opportunity to one day bring her back in some way, so that was a way for me to leave an opening if I ever got the opportunity.
Fast forward to now, many sessions later, one my players said they have a friend who would like to come to our session and just watch as they've never played D&D before. I immediately want to seize this moment and suggest instead that she come in and play a guest character.
Now the character had a really cool backstory of her grandparents (who raised her) would tell her the legend of Acererak coming to Omu and destroying the trickster gods. This became the obsession of her character to find out Omu and what really happened and how this related to the disappearance of her grandparents.
At this point in the campaign, my players just happen to be in Omu and in the process of trying to get all the keys to open the tomb, so here's what I want to do and I would like some help fleshing this idea out.
I'd like the returning character to stumble upon the party while they explore Omu (that way I can get the new player in action right away at the start of the session). But, in the time that this returning character was away, she's changed. She's now convinced that Acererak is the key to bringing back her grandparents, the most important people in her life ever, so she's become an oath breaker paladin. She has more lore of what Acererak is really up to (through her research) and wants to actually help him bring forth the Antropal... just like Acererak deceived Ras Nsi to guard the tomb, Acererak has done sort of the same with this guest character bc he's aware of the party bc of information from the Sewn Sisters (which have appeared to the characters in dreams several times through their exploration).
Now, I've talked with the player about the idea of playing essentially a, now, evil character that will probably end in a PvP battle. They're good with it. But, I told them to please let me know at any point if they don't like an idea bc my goal is always to ensure that everyone at my table has a good time.
My question is, how best to implement this for the biggest emotional gut punch and shock value to my core players (the players that bonded with the original PC) as possible... while recognizing the limitations of a first time DnD player? Or is this just simply a bad idea?
PS. When the original player left the campaign, I got permission to keep her character alive and potentially use her as an future NPC.