Does Allah take revenge on a believer who worships and fears Him by afflicting him with a serious disease like AIDS due to committing adultery?

Fornication is one of the gravest major sins and immoral acts from which Allah Almighty has warned, saying: "And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way." Nevertheless, Allah's mercy is vast, and He accepts the repentance of those who repent, as He Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" Your friend must offer sincere repentance and regret his action, for Allah transforms the bad deeds of the repentant into good deeds, as stated in Surat Al-Furqan. If he repents sincerely, then "The one who repents from sin is like one who has no sin." He must also conceal his sin and not inform anyone of his disobedience, in compliance with the saying of the Prophet (peace and blessings be upon him): "Avoid these abominable acts... and whoever commits any of them, let him conceal it with Allah's concealment and repent to Allah Almighty."