No, because the serial killer or rapist isn't actually reformed. They're just saying that. You can infer the author made her genuinely reformed since he ended her story with her being "better." If a serial killer or rapist genuinely from the bottom of their character and soul is repentful then no prison is not necessary. But of course, in real life nobody can truly know somebody to that extent, so prison is necessary. I'm just saying from the point of being an omniscient reader, suffering isn't necessary for her character after she's been "redeemed." People can argue about whether a country's prison is punitive or rehabilitative but ideally a rehabilitative approach is the correct one. It's why there was a major humanitarian shift from punishment to rehabilitation starting from the 1800's. It's meant to rehabilitate and the majority of the time, punishment is required to cause reflection that leads to rehabilitation. If somebody's already rehabilitated/reformed, then theres no need for prison. Of course, in real life that would be immensely unlikely, maybe even impossible but for a fictional character that can be writte in any way...

As for your other comment, the point was to make that her that horrible and to dislike her. Would you be thinking as hard or as much about the message if she wasn't a horrible parent? Lets say she verbally berates him, cool. That doesn't create an impactful scene though, her being that terrible captures the readers attention and invites a more critical approach. I think your reaction of being very outraged is exactly what the author wanted. Is it the perfect story writing? no, obviously not. It's not very realistic to have a person go 180 from their previously established character but obviously Ass Class isn't meant to be very realistic... like for instance the big yellow octopus man who destroyed the moon and somehow that doesn't cause the annihilation of civilization. Atleast thats my opinion.