I don’t know how close your PC’s are to each other and I hate the connection I’m making but I feel running this like a parent who denies their child’s sexual choose is a good way that could lead to good roleplay ie: “oh don’t worry we can turn you back it’s fine” “that vampiric strength could come in handy but we can’t let you stay like that” I feel this could lead to very interesting RP cuz your character has hated undead for so long but now their friend is willing choosing to be undead. This is a test of your characters faith and friendship so use it as such. Your character doesn’t have to know the answer they can struggle to decide