Is mentioning someone's injustice to another person considered forbidden backbiting?

The basic rule regarding mentioning a Muslim in their absence with something they dislike is that it constitutes forbidden backbiting, unless the person mentioned is an oppressor. In this case, the wronged person may mention their oppressor with the intention of repelling their oppression or reclaiming their right, or for retribution, without lying or wronging others. However, refraining from doing so is better. As for describing someone as an oppressor when they have not wronged you, but rather wronged your friend or relative, this is considered forbidden backbiting, unless it is for the purpose of warning and offering advice.