Whatever "treatment" they did to her, it accomplished the exact opposite of what it should have done and there's no telling if any of it is reversible. If the soul itself is heavily influenced by what happens to their living vessel throughout life, redemption could be near impossible for Niffty if she is unable to stop being a danger to herself and others. However, if someone helped her relearn some of those social skills, mainly impulse control so she's less of a danger, then it might be possible provided there was a reason she wants to go to Heaven. In most cases it would take Alastor and Husk getting redeemed first before she would even think of trying. Wherever they go is where she wants to be.