Brother having affair with friend's wife
18-10-2017 | IslamWeb
Question:
My friend has been diagnosed with a severe bone disease that is non-curable. I have a twin brother who has become very close with my friend's wife and is having physical relations with her. I have seen pics and texts of their chats. Now I am in a dilemma as to what I should do. Should i tell my ailing friend about this? (Doctors are saying this is a terminal disease though it is not cancer.) By not saying this, I feel guilty as well. I have spoken to my friend's wife and told her to stay away and be faithful, but she instead poisoned my twin brother's ears about me. If anything happens to my friend, I will be guilt-ridden until my last breath. Please suggest a way for me to handle this. My brother is also gifting her and her child expensive phones etc.
Answer:
All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
We ask Allah, the Exalted, to bless this man with recovery. You should supplicate Allah in his favor frequently and advise him to supplicate Allah and to recite Ruqyah (Quranic healing); verily, Allah is the True Healer and there is no healing but His. Jaabir 
narrated that the Prophet, sallallaahu ʻalayhi wa sallam, said, “There is a remedy for every ailment, and when the correct remedy is applied, the ailment is cured with the permission of Allah, the Exalted and Glorious.” [Muslim]
If it is proven that your brother is having an affair with this man's wife, then that is a heinous sin. Islam prohibited such illicit affairs. Allah, the Exalted, says (what means): {...desiring chastity, not unlawful sexual intercourse or taking (secret) lovers...} [Quran 5:5] The sin is graver when the woman is married. In addition, this woman's engagement in this sinful relationship is disgraceful infidelity on her part.
Your duty is to give them advice and remind them of Allah, and this should be done in a gentle, kind manner so that the advice would be effective. If they heeded your advice and refrained from their sin, then all praise be to Allah; otherwise, you could threaten to expose them. If this is effective in deterring them, then it is enough; otherwise, you should hint to her husband lest his honor be defiled and his wife attributes a child to him that is not his. If the hinting is sufficient, then it is enough; otherwise, if there is a need to be explicit, then you should only inform him of his wife's affair without accusing her of committing adultery lest you become a slanderer, which is a serious sin that entails a punishment in this life and in the Hereafter.
Allah knows best.