Backbiting is a major sin that Allah condemned with a severe image, comparing it to eating the flesh of one's dead brother: 'Would any of you like to eat the flesh of his dead brother? You would hate it' (Quran 49:12). The Prophet ﷺ defined it precisely: 'Backbiting is your mentioning of your brother with what he dislikes.' When asked what if the fault is really in him, he replied, 'If what you say is in him you have backbitten him, and if it is not in him you have slandered him.' Islam therefore forbids speaking about a person's genuine faults behind their back, except in a few legitimate cases such as seeking help against oppression or warning someone of harm.
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.