Praise be to Allaah and Blessings and Peace be upon Allah's Messenger

There are two important issues in her question.

 Her husband's affair with his friend's wife: Firstly, we will not get into the details of how deep the relation has gone. We leave that matter to Allaah. But if she has discovered her husband's relation who confessed about it, then:

 Even if she has forgiven him, he is a sinner before Allah, because he has disobeyed Him. He has to repent before Allah very sincerely, as the Qur'an declares: "O you who believe, repent before Allah with a sincere repentance." If he has sincerely repented before Allah and is firm never to repeat that sin, for that, the Prophet (blessings and peace of Allah be upon him) has said, "If a person repents sincerely before Allah, it is as if he has not committed that sin at all". Therefore, after that, it is not allowed for others to discuss about his sins or highlight it. Also, he should not disclose his sins after repenting sincerely.

  In our times, people apologize before their wives but not before Allah, and there are also some who boast about their sins amongst friends. It is a big sin not to feel bad about one's sins, rather publicly display the sins as achievements. There is a Sahih Hadith in Sahih Muslim, which quotes the Prophet, peace be upon him, "All the sinners of my Ummah will be forgiven except the 'mujahiroon'". The Sahaba inquired, "Who are the 'mujahiroon'?" He replied, "He who commits sins secretly but discloses them before people".

  Another Hadith comes from a noted companion, Ubada bin Thabit, who quotes the Prophet, peace be upon him, "If such and such sins of a person come before us and he is proved guilty, then we will apply punishment upon him, and this punishment will be an expiation for his sins, and he will be exempted from being punished from that particular sin on the Day of Judgement. But, if his sins are covered from the people, then it is between him and Allah, either He will punish him on the Day of Judgement or pardon him.

  That is why, the Prophet, (blessings and peace of Allah be upon him), always preferred that people should not discuss their sins with others, because it can encourage others too. There is an incident of Mai'de Aslami (may Allah be pleased with him), which occurred during the times of the Prophet, peace be upon him. He committed an act of zina, but before reporting it to the Prophet, peace be upon him, he went to his boss Hazzal and confessed his sin. Hazzal told him to go and confess before the Prophet quickly,so that he can be forgiven. Therefore, Madie Aslami went to the Prophet and said, "O Messenger of Allah! I have committed this sin". The Prophet turned his head away and ignored it. But Mai'de Aslam went on confessing repeatedly and insisted that he be purified of his sins and continued to explain about it in detail. The Prophet kept on avoiding his confession, but finally, when he confessed it firmly, the Prophet told others that he might have been under the influence of alcohol, because of which he might be behaving in this manner. He asked people to smell his mouth. The man said, "I am not drunk but I am confessing my sin truthfully".

  So, the Prophet, (blessings and peace of Allah be upon him), commanded him to be stoned to death. When he was being stoned, Mai'de Aslami could not bear it so he requested to be taken before the Messenger of Allah, (blessings and peace of Allah be upon him). But, since the punishment was imposed, he was stoned to death. After that, the Prophet, (blessings and peace of Allah be upon him), announced two important issues:

1. Among the people who were stoning him to death, there was one whose cloth got stained as the blood of Mai'de Aslami got sprinkled on his cloth when he threw a stone. So he said, "The sinners blood is on my clothes". The Prophet, peace be upon him, heard this and said, "It may be that you think he died a bad death, but I see him enjoying in the rivers of Paradise because he has repented sincerely and got the punishment in this world itself". Therefore, if a man receives punishment in this world, it should not be discussed.

2. The Prophet, peace be upon him, then called Mai'de Aslami's owner, because it was upon his insisting that Mai'de came and confessed his sins, and told him, "O Hazzal! You made a big mistake. Had you encouraged him to repent before Allah instead of asking him to uncover his sin, it would have been better. If the sin is uncovered before us, we have to punish him".

Now, the second part of the question: She feels very bad whenever she recalls his confessions and does not like to be with him.

1. The husband is confessing while her friend, the lady in affair, is refusing to accept it. Then there cannot be any punishment. For such sins,there needs to be at least four witnesses before a ruling is imposed. On the other hand, Islam has not allowed us to dig up hidden sins.

2. It is the nature of a woman to recall such things and feel bad. It is gheerah (self respect, jealousy, modesty) that makes her feel bad whenever she remembers it. Therefore, she should make dua to Allah to enable her forget the incident because, since he has repented, it is better to leave the matter and not be suspicious and keep on pursuing the issue. She should not fail in fulfilling her duties towards her husband.

 When the Prophet, (blessings and peace of Allah be upon him), sent a proposal of marriage to Umm Salama, she refused to accept it in the beginning, citing two reasons, "I have a daughter from my first husband Abu Salama. I would marry only if my daughter is allowed to stay with me. But it could be possible that my young daughter, Zaynub, might disturb the Prophet while he is resting. It could also be possible that at times, I may be more attentive towards my daughter instead of the Prophet". She also recalled the gheerah of her husband and felt bad to marry another man after his death.

  Her second reason was her gheerah as a woman. A woman may not be able to tolerate her husband with other wives. Even though Aisha (may Allah be pleased with her) was among the best women in the world, yet she was not without this gheerah. There was this sense of gheerah among all the wives of the Prophet because they were human.

So, the Prophet offered to treat her daughter Zaynub as his own daugther, and with regards to her gheerah, he said I will pray to Allaah to solve it.

Therefore, my advice to you is do not discuss the past incident with your husband. Rather, make Dua to Allah to remove that feeling from your heart which stops you from performing your duties as a wife. After you have forgiven your husband, it is better that you don't carry the grudge in your heart. There are two terms in the Quran, 'fa'afooanhu'  and 'was'fahu' - 'tolerate him' and 'forgive him'. To tolerate means not to avenge and to forgive means not to recall the incident. Thus, Qur'anic teachings of forgiveness include forgiving outwardly as well as inwardly.

 "Do tolerate the shortcomings after pardoning and do not harbour hatred in your heart after pardoning and further, do you not want Allah to forgive your sins?"

So, if you know that your husband has many qualities that are good along with a few sinful deeds, then focus on his good qualities, else Shaytan will cause disruption in relations and in future, things can get worse. Shaytan can even incite you to break off your marriage. So, forgive for the sake of Allaah. Every human being commits sins and we should pray for his forgiveness.

 Jazak Allahu Khairan

And Allaah knows best.