Is it obligatory to condemn the backbiter every time he backbites, knowing that most of their speech is backbiting, and does the sin fall upon the silent one or the one who is in doubt as to whether the situation constitutes backbiting or not?

Censuring one who backbites is obligatory, but it should not reach the point of obsessive doubt. Not everything mentioned is considered backbiting, such as the grandfather's saying and the one who called the driver, or the uncle's saying which is not mockery unless it is directed at you. Also, mentioning a fault with the intention of warning is not backbiting, nor is mentioning a transgressor or disobedient person for what they are. One must learn the situations where it is not considered backbiting, such as speaking about an unknown person whom the listeners do not know, as in the Hadith (definition) of Umm Zar'. If these rules are known, it becomes clear that much of what is condemned is not backbiting that necessitates censure. Precaution and guarding one's tongue are good, and it is preferable to remind those present of Allah and to speak about what benefits them.