Is conveying the discussion that took place about my friend's sister, which caused a problem, considered backbiting and a sin?

It is obligatory to defend a Muslim's honor, for the Prophet, peace and blessings be upon him, said: "Whoever defends the honor of a believer, Allah will defend his face from the Hellfire on the Day of Resurrection." The general rule is that backbiting, which is conveying speech between people, is forbidden. However, if one is compelled to inform others for the purpose of warning, then the matter should be mentioned without identifying the speaker. If identifying the speaker becomes necessary to avert harm from the person themselves, their property, or their honor, then they may be mentioned to the extent of the necessity.