The only thing you should do behind someone’s back, is make Dua for them.

The Prophet (pbuh) defined backbiting when he said:
“Do you know what is meant by backbiting?” They said, “Allah and His Messenger know best.” He said, “To say something about your brother which he dislikes.” One asked, “Even if what I say is true about my brother?” He replied, “If such defects you say are true about him, then you have backbitten him, and if he doesn’t have what you say, then you have committed slander against him.”
(Related by Muslim, Abu Dawood, and Tirmidhi)
Hence, it is not allowed to talk about somebody in his/her absence, even if what was narrated was true. If it was not true, then it would be a bigger sin and it is called Buhtan (false accusation).

The Messenger of Allah (SAW) said: “Whoever believes in Allah and the Last day, LET HIM EITHER SPEAK GOOD OR KEEP SILENT”

It is important to note that to find other’s faults or to talk sarcastically or to defame a person is called ‘Lumz’. In the Qur'an, Allah (SWT) states: “Do not defame one another.”
(Surah Al-Hujurat, 49:11)

May Allah (SWT)protect our tongues from backbiting and defaming one another.