Her body still has a living soul in it, so as a necromancer, it's not suitable for his experiments.
Seriously, though, I imagine this is related to her past and what made her want to become independent and not rely on anyone. So, she'd be just testing to see if Asamura is a person (of low morals) who would do such a thing, instead of respecting their status as step-siblings and help her normally (I say saving her from death/serious injury is already helping quite a lot and in high morals). But who knows what her intentions are. Now that I think about it, maybe she asked this question from someone else before and that's where the rumours about prostitution came from.