Zina (adultery) is nothing but evil. But with a married woman it is a greater sin because that is a transgression against the husband’s honour. Allaah says concerning zina (interpretation of the meaning): 

“And come not near to unlawful sex. Verily, it is a Faahishah (i.e. anything that transgresses its limits: a great sin), and an evil way (that leads one to hell unless Allaah forgives him)”

[al-Isra’ 17:32]

The Prophet (peace and blessings of Allaah be upon him) said: “If a man commits zina, faith will come out of him until it becomes like a cloud over him, and when he stops, his faith comes back to him.” Narrated by Abu Dawood, 4690; al-Tirmidhi, 2625; classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

Ibn Hajar al-Haythami said in al-Zawaajir ‘an Iqtiraaf al-Kabaa’ir (2/138): Thus it is also known that there are different categories of zina. If it is done with a non-mahram woman who has no husband it is a serious matter. It is more serious if it is with a non-mahram woman who has a husband; it is more serious still if it is with a mahram. The zina of a previously-married person is worse than zina of a virgin, which is indicated by the fact that the punishment is different. And the zina of an old man who is of sound mind is worse than the zina of a young man, and the zina of a free man or a knowledgeable man is worse than the zina of a slave or one who is ignorant. End quote. 

The expiation for committing zina with a married woman or any other is sincere repentance that fulfils all the conditions of repentance. That involves giving up the sin completely, regretting what one has done and resolving never to go back to it. Whoever does that has repented to Allaah, and whoever repents, Allaah will accept his repentance and turn his bad deeds into good deeds, as Allaah says (interpretation of the meaning): 

“Know they not that Allaah accepts repentance from His slaves and takes the Sadaqaat (alms, charity), and that Allaah Alone is the One Who forgives and accepts repentance, Most Merciful?”

[al-Tawbah 9:104] 

“And those who invoke not any other ilaah (god) along with Allaah, nor kill such person as Allaah has forbidden, except for just cause, nor commit illegal sexual intercourse and whoever does this shall receive the punishment.

69. The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace;

70. Except those who repent and believe (in Islamic Monotheism), and do righteous deeds; for those, Allaah will change their sins into good deeds, and Allaah is Oft‑Forgiving, Most Merciful”

[al-Furqaan 25:68-70]

Al-Bukhaari (4436) and Muslim (174) narrated from Ibn ‘Abbaas (may Allaah be pleased with him) that some people from among the mushrikeen who had killed and killed a great deal, and committed zina and done it a great deal came to Muhammad (peace and blessings of Allaah be upon him) and said: “What you are saying and calling us to is good, if only you could tell us that there is any expiation for what we have done.” Then the words “And those who invoke not any other ilaah (god) along with Allaah, nor kill such person as Allaah has forbidden, except for just cause, nor commit illegal sexual intercourse” [al-Furqaan 25:68] and “Say: O ‘Ibaadi (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allaah” [al-Zumar 39:52] were revealed. 

Whoever falls into zina must hasten to repent to Allaah and conceal himself with His concealment, and not disclose what he has done, because the Prophet (peace and blessings of Allaah be upon him) said: “Avoid this filthy thing that Allaah has forbidden, and whoever commits any sin of that nature let him conceal himself with the concealment of Allaah.” Narrated by al-Bayhaqi and classed as saheeh by al-Albaani in al-Silsilah al-Saheehah, 663. 

Muslim (2590) narrated from Abu Hurayrah (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “Allaah does not cover any slave in this world but Allaah will also cover him on the Day of Resurrection.” 

And Allaah knows best.