We put this question to Shaykh ‘Abd-Allah ibn Jibreen (may Allah preserve him), who answered as follows:
This is gheebah, because there is no benefit in complaining to other women in this manner. The employer has to discipline and train her servant properly. If she wants to consult with knowledgeable people (if there is a woman among them who has experience and knowledge of such matters) about how she can discipline and train her servant properly, then it is OK to mention the servant’s faults (because there is a legitimate need to do so).
And Allah knows best.