A redemption arc can be good, BUT it has to be done correctly. She can't be forgiven, but she can move on and be a better person. Hopefully the author does it well.
Technically there is only 1 way to be forgiven, and that is basically just staying away from her ex boyfriend for the rest of her life. If he one day approaches her, so be it, but accepting and understanding her cruelty is really her only path to salvation.
Rather than a redemption arc, seeing Hanabi's reaction to this new girl (her sister?), I take it she's probably the root of all of Hanabi's delusional ass thoughts.