Two things can be true at once.
Hanabi can be a godawful person who made cruel choices and directly contributed to years of abuse and trauma for the MC.
And she can absolutely not deserve what's being implied as imminent for her future at the hands of the delinquent.
This isn't "justice", and I'd hesitate to call it "retribution" - it's just exploitation.
If this were really about her 'paying for her crimes' against Souma, then it would be taking the form of something that actually serves as resolution for Souma. This? Doesn't do that. It just violates Hanabi in a manner that has no connection to the one she abused, with no sense of balancing the scales back in Souma's direction.
That is why I see this as problematic. There's nothing for Souma to gain from this, especially as I do not see him as the type that would find satisfaction in knowing his ex/abuser was in danger of being raped/trafficked "to protect him" or whatever.
Unfortunately it'll likely fall to Souma to save her now, too - which funnily enough, I think would go closer to enacting justice for him, since he now gets to clean up her mess and show her up in how selfless he can be, going to bat even for someone like her.
Hopefully that realization fully drives home what she'd done to him and what he's become in spite of that, and maybe she can find a more constructive way to serve penance for her past.