Person A committed a major sin. However, he sincerely repented.

Answer

Allah Ta'ala says in the

Qur'aan Majeed: "Say (O Muhammad!) to My servants who have oppressed themselves (with sins): 'Do not despair of the mercy of Allah, for verily, Allah forgives all sins."

Rasulullah (sallallahu alayhi wasallam) said: "A sincere repenter of sins is like one who has no sins." In another Hadith, Rasulullah (sallallahu alayhi wasallam) said: "He who taunts a brother (i.e. a Muslim) with a sin (for which he has made Taubah), he will not die until he himself has not committed that sin."

Person B has committed an evil sin by publicizing the sin of person A. Whilst person A has been forgiven by Allah Ta'ala, the sins are piling up on person B who is guilty of the heinous haraam act of exposing a brother Muslim. Person B should reflect on his own sins. He is not Ma'soom (sinless). He is soaked in sin from head to feet. If he reflects he will realize that he has many skeletons in the cupboard and cans of worms. If Allah Ta'ala decides to expose and disgrace him by revealing his skeletons and cans of worm, then he will be thoroughly humiliated.

Person B should understand that Allah Ta'ala is As-Sattaar, i.e. He is the Concealer of the sins of people. When Allah Ta'ala is Saatirul Uyoob (The Concealer of sins), from whence did person B arrogate to himself the right to expose and humiliate a brother Muslims, especially since the sinner has sincerely repented. Taubah has purified person A of his sin, and person B has become soiled and polluted with his kabeerah sins of publicizing person A's non-existent sin. It is non-existent because Taubah has eliminated the sin from even the Book of Deeds, and from even the memory of the Recording Angels.

Person B should fear and understand that if he will not desist from his heinous sin of publicizing the sins of others, and if he will not seek person A's pardon, and make Taubah, then before he dies Allah Ta'ala will induce him to commit such a vile misdeed which will thoroughly disgrace him in this dunya. And, the punishment and disgrace of the Aakhirah, are other totally unbearable issues.