He didn't think that through very well. A B rank mercenary who kills for fun isn't the best body to choose for your friend who was slaughtered. On the bright side, she should be able to do anything Nasha was able to do. On the more realistic side, since they have no idea what Nasha's abilities were in the first place, it's almost impossible to guess how she will learn how to use this new body. Maybe he will BS something like "the body still remembers how to move even if the soul does not". Plus he'll need to learn the limits of his magic. If he passes out when using too much power then that's going to be an issue as well.