Nothing about her could, can, or will ever justify her actions. I don't care if she was raised in an Eastern European orphanage and forced to pimp herself out for money to buy food, it doesn't excuse what she did to Momo.
If someone is that starved for attention, one could always..I don't know, join a damn school club or be an actually nice person.
And even if she did change, I would never have forgiven her at all. Ever. I wouldn't forgive nor forget, because there are some people who don't deserve forgiveness when something as simple as human decency and not meddling in other peoples' affairs should be unspoken and obvious. Those who do violate such rules of proper conduct --excluding those with actual mental illnesses that would explain such transgressions--shouldn't be pitied or sympathized with.