Alright here me out. She's a bad person, but she could be a better person so she doesn't hurt anyone else in the future. And the only way to rehab people like this is to show them by example and hold them to the straight and narrow.
Sakine doesn't have to forgive her. That's between them, not us. What matter is how she grows or doesn't grow as a person. Casting her aside for a mistake, a big big big one, when she was like 13 is not the correct choice to make if you want to live in a functional society. Rehabilitate her or your part of the problem too. Isolating her and leaving her to her bad choices only creates more victims of abuse in the future.
The cycle of abuse and broken people from broken homes ends with us as a community learning them some manners. And if you think she can't be redeemed for something she did as a 13 year old, you're being unreasonable. Furthermore, it's still up to sakine whether she forgives her. Abandoning a child to bad behavior does nothing good for our world.
All that said, it doesn't make outing her girlfriend any less disgusting.
It's a tough thing, for sure.
Because people in Sakine's position
should be affording that opportunity, if they truly wish to move forward and grow and become the sort of person who will break away from that cycle of abuse and be kinder toward others,
and seek to make amends/serve some form of penance for the hurt they've caused others. It's as you say--the cycle ends when everyone steps forward and ends it, and that includes extending a hand and heart to the perpetrators, as they're quite often victims themselves who could change if afforded patience and grace.
It's just also not fair to those hurt by them, to be the ones whose hands and hearts are put forth. It's
great when that can happen and people can come together, but demanding it of them does them a disservice, and even if it would be in everyone's best interest to set personal history aside for the sake of aid...well, I wouldn't fault Sakine in the slightest if she decided "No" and refused to allow Hibiki back into her life in any way.
And at the same time, I both want her and Sakine to make up and I don't, for those reasons. It's hard to write a story like this in a realistic, grounded fashion, because a realistic interpretation would involve getting adults and professionals involved and likely years of therapeutic assistance and counseling. That can be shown in the narrative, but it would require time skips, and also just ....isn't "sexy" like having a big lesbian makeout session that magically fixes everything.
Someone else here said it well--it's effectively wish fulfillment to have a happy ending for Hibiki, but otherwise it's her being ousted and walking away, fully without any support base, and that in turn would be immensely unsatisfying after she's come to full terms with how shitty she's been and how much regret she carries. The stage is set for her making amends and putting in the effort to change her ways and redeem herself. That not happening...well, it would just feel bad.
I think we as readers should be able to understand why she did what she did, and also
not condone what she did; but I think I'm not alone in hoping she gets some sort of good outcome from all of this.