She committed zina with a non-Muslim during the day in Ramadan a few years ago, then she regretted it. What should she do?

Firstly: 

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.”

[Al-Furqaan
25:68-70]

“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.”

[az-Zumar
39:53]. 

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. 

Secondly: 

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. 

They replied: 

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.