Zina is a major sin and one of the worst crimes. Allaah says (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)”
“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”
It makes no difference whether zina is committed with a Muslim woman or a non-Muslim.
As for the punishment for zina in this world, Allaah has enjoined the hadd punishment for it. Allaah says concerning the hadd punishment of an unmarried zaani (i.e., one who has not been previously married) (interpretation of the meaning):
“The fornicatress and the fornicator, flog each of them with a hundred stripes. Let not pity withhold you in their case, in a punishment prescribed by Allaah, if you believe in Allaah and the Last Day. And let a party of the believers witness their punishment”
As for the one who has previously been married, the punishment is to be stoned to death, as it says in the hadeeth narrated by Muslim in his Saheeh (3199) from the Prophet (peace and blessings of Allaah be upon him), who said: “For a previously married person with a previously married person, one hundred lashes and stoning.”
In this case too, it makes no difference whether zina is committed with a Muslim woman or a non-Muslim.
The seriousness of this crime is not limited only to immediate punishment in this world, but the punishment in the Hereafter is harsher and more severe. It says in the hadeeth narrated by al-Bukhaari (7074) from Samurah ibn Jundub (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “Last night two persons came to me and woke me up, and set off with me. … We went on and we came to something like a tannoor oven, in which there were shouting and voices. We looked inside and we saw naked men and women, towards whom flames were coming from the bottom of the oven. When the flames reached them they made a noise. I said to them [the two angels], ‘Who are these people?’ … They said to me, ‘We will tell you… the naked men and women in the structure like a tannoor oven are the adulterers and adulteresses.”
What the one who has fallen into this major sin must do is repent sincerely to Allaah, and keep away from everything that may lead to this haraam or cause him to return to it. Allaah rejoices over the repentance of sinners and accepts it from them. Allaah says (interpretation of the meaning):
“Say: O ‘Ibaadi (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allaah, verily, Allaah forgives all sins. Truly, He is Oft‑Forgiving, Most Merciful”
Ibn Katheer (may Allaah have mercy on him) said: This verse is a call to all sinners, kaafirs and others, to repent and turn to Allaah. It tells us that Allaah forgives all sins for the one who repents to Him and turns away from them, no matter what they are, even if they are many and are like the foam of the sea. End quote from Tafseer Ibn Katheer (7/106)
And Allaah knows best.