Ruling on one who committed zina with a non-Muslim woman when he was not married
Question

                What is the ruling on committing zina with a non-Muslim woman when one is not married?.
Praise be to Allah.Firstly: 
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)”
[al-Isra’ 17:32] 
“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”
[al-Furqaan 25:68-69]
It makes no difference 
    whether zina is committed with a Muslim woman or a non-Muslim. 
Secondly: 
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”
[al-Noor 24:2]. 
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”

    [al-Zumar 39:53]
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.