You can try explaining to her that a critical negative attitude towards others harms both. It only strengthens this tendency. Everyone has their faults and it is best not to complaint of things we cannot change. In any case, in human relationships, it is best to connect with the best part of another and not pay attention to the real or unreal faults. But it is unlikely that explaining will much help her. People who are overcritical of others often do so out of a compensatory mechanism to cover up their sense of inferiority. It will be best therefore to either show her her own beautiful parts and strengths while ignoring this critical attitude.
She may feel bad if you don’t much pay attention but if alongside you show her the positive things in her, she will surely slowly understand your good intent.
Affectionately,
Alok Da
