What is the expiation for one woman slapping another on the face due to verbal abuse, given that it is not possible to seek forgiveness from her?

A woman must repent to Allah for the injustice she committed. Repentance has four pillars: remorse for what has passed, abandoning the sin immediately, resolving not to return to it in the future, and seeking absolution from the injustices committed against people. If the wronged person cannot be found, and the injustice was not financial, then she should increase her istighfar (seeking forgiveness) and supplications for him, and intend to seek his absolution if she finds him, even in the Hereafter. If the injustice was against a deceased person concerning their honor, then she should intend to seek their absolution or seek forgiveness for them until they are pleased with her.