Assalamu aleykum.My mother always give complain of her boss to me when i can't settle the score between her and her boss.Is this a form of backbiting?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The basic ruling is that mentioning the faults of Muslims in their absence is an act of prohibited backbiting.

As for the backbiting of the wronged person against the person who wronged him and his complaint against him, then it is an exception from this basic ruling, based on Allah's saying (which means): {Allah does not like the public mention of evil except by one who has been wronged. And ever is Allah Hearing and Knowing.} [Quran 4:148]

However, many scholars hold the view that the above-mentioned exception is not general but rather restricted to doing so to the person who has the ability to bring the wronged person’s rights back. See, for example, Imam An-Nawawi’s Riyadh As-Saaliheen.
Whereas Ibn Taymiyah  is of the view that is allowed to backbite the wrong-doer under all circumstances, viewing this as a kind of retribution. In his Minhaaj As-Sunnah, he says, “The lawful part of this (i.e., backbiting) is what is declared lawful by Allah and His Messenger, that is, the act the serves as some sort of retribution and bringing justice back, such when the complainer says, 'So and so hit me, took my money, denied my right, etc.' Allah, the Exalted, says, (what means): {Allah does not like the public mention of evil except by one who has been wronged. And ever is Allah Hearing and Knowing.} [Quran, 4:148]”

Allah knows the best.