There is no doubt that zina (fornication) is a major sin and is something that incurs the wrath and anger of Allah. If zina was committed with a non-Muslim, the sin is greater, and if that was during the day in Ramadan, that is even worse. If the person who committed the sin does not repent sincerely from the bottom of his heart, then his loss is complete.
Allah, may He be exalted, says (interpretation of the meaning):
And those who invoke not any other ilah (god) along with Allah, nor kill such life as Allah has forbidden, except for just cause, nor commit illegal sexual intercourse and whoever does this shall receive the punishment.
The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace;
Except those who repent and believe (in Islamic Monotheism), and do righteous deeds, for those, Allah will change their sins into good deeds, and Allah is Oft-Forgiving, Most Merciful.
Say: O My slaves who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allah, verily Allah forgives all sins. Truly, He is Oft-Forgiving, Most Merciful.
Despite the abhorrent nature and severity of the sin and the fact that the one who committed it is exposed to the wrath and anger of Allah, the mercy of Allah precedes His wrath, so He does not hasten to punish His slave; rather He opens to him the gate of repentance and gives him time to repent. If he repents and is sincere, Allah will forgive him, no matter what his sin was.
What this young woman must do is be sincere towards Allah in her repentance, regret having committed this abhorrent action, resolve never to commit this sin again, and do a lot of righteous deeds. She should turn to her Lord, thinking positively of Him, being sincere towards Him and repenting fully to Him. Allah accepts the repentance of His slaves and grants pardon for bad deeds.
The one who committed zina during the day in Ramadan has to, along with repenting, make up that day's fast and offer the severe expiation, which is to free a slave; if he cannot do that, then he must fast for two consecutive months; and if he cannot do that, he must feed sixty poor persons.
See also the answer to question no. 39734
The one who did that several years ago and has not made up the fasts that he missed also, in addition to the above, has to offer expiation for delaying the making up of that day's fast, which is to feed one poor person; the amount to be given is half a saa of the local staple food, wheat or rice and so on. That is equivalent to one and a half kilograms.
The Standing Committee was asked:
What is the ruling on one who committed zina during the day in Ramadan? That took place at the time when he was ignorant and now he is asking Allah for forgiveness.
The questioner has to repent and must offer expiation, which is to free a slave; if he cannot do that, then he must fast for two consecutive months; if he cannot do that, then he has to feed sixty poor persons. He also has to make up the fast of that day on which he broke the fast, and he has to offer expiation for the delay in making up that day's fast, which is one and a half kilograms of wheat.
End quote from Fataawa al-Lajnah ad-Daa'imah, 9/255
And Allah knows best.
Reproduced from Islam QA