I have also had this issue.
The first time I did this, the sim turned back before he left, but I did it with Lily Feng, and she left before becoming a normal sim again, and now she is stuck as a ghost. MCCC does not have an option (that I can find) to remove the ghost flag. I have tried to use cheats, but all ghost flag cheats are played sim centric.
I have tried editing Lily in CAS and saving her, still a ghost. She was pregnant when she turned and now the pregnancy is gone with no kids. Basically she just... died? IDK. I would really like to fix this some how. She and my sim were best friends and now it just feels awkward for my sim to get spooked every time she visits, and spending 8000 satisfaction points on "Brave" just to talk to one sim feels stupid.