Well I realize the problem is that as soon she takes her soul back from him I can't see her just walking away, the moment she could feel fully she'd be like "omg this is terrible for him how can we fix this." Buuuuut, if you want him to run around soulless/mephisto-soul-evil-influenced for awhile and we can think of a reason why she wouldn't (or couldn't) immediately restore some soul/humanity to him, then I am totally up for that!