If he repents and seeks forgiveness, the mercy of Almighty Allah is expected to reach him. He shall not be subject to any punishment, God willing.

Repentance, which is regretting the committing of sins and the determination to refrain from them in the future, is one of the means of the mercy of Almighty Allah. Seeking forgiveness from Almighty Allah and continuing to do so whenever one remembers the sin helps the purification of the heart.
Performing the good deeds leads to the cleanliness of the heart as well. Almighty Allah said: “Good deeds annul ill-deeds” (11:114).

It has been reported that Umrah is expiation from sin and this is a manifestation of God's mercy and kindness to His slaves when He legislated for them certain acts of obedience that lead to forgiveness of sins. This is a common message in the religious texts. Despite that, a devotee should be afraid of his sins so he may become closer to Allah's mercy and acceptance, and this may be an encouragement to perform more good deeds and avoid committing sins.
Almighty Allah said: "And those who give that which they give with hearts afraid because they are about to return unto their Lord; these race for the good things, and they shall win them in the race"(23:60-61).

All sins can be forgiven by sincere repentance. Almighty Allah said: “Despair not of the mercy of Allah, Who forgives all sins. Lo! He is the Forgiving, the Merciful”. Yes, it is recommended to seek forgiveness whenever the committed sins are remembered but should not turn into psychological complication.

Getting married to the adulterer does not decrease the sin. What does is repentance and repeatedly seeking forgiveness. Almighty Allah said: “Say: O my slaves who have been prodigal to their own hurt! Despair not of the mercy of Allah, Who forgives all sins. He is the Forgiving, the Merciful” (39:53).

Repentance erases the sin and leads to forgiveness. It is certain that some good deeds lead to erasing the sins as Almighty Allah says: “Good deeds annul ill-deeds” 11:114. However, we cannot give a general criterion for that. For a believer, it is better to combine repentance, seeking forgiveness and performing good deeds so he may be closer to Almighty Allah’s mercy and forgiveness.

Repentance is regret and determination to avoid committing sins in the future. Being in the state of purity, prayers or fasting is not part of it. Yes, it is recommended to repeatedly seek forgiveness whenever sins are remembered. A devotee is expected to have high hope in Almighty Allah’s acceptance of the repentance and the forgiveness of sins. Almighty Allah said: “O my slaves who have been prodigal to their own hurt! Despair not of the mercy of Allah, Who forgives all sins. Lo! He is the Forgiving, the Merciful.” (39:53).

The point that this verse express is that if one refrains from committing big sins, this will become a cause for the mercy of the Almighty Allah in the forgiveness of the small sins. This does not mean that one can commit small sins and disobey Almighty Allah. Furthermore, according to traditions the belittling of committing small sins or persisting in committing them is considered to be a major sin.

There is no special Kaffara prescribed for this act. One should repent and seek forgiveness.

You should continue to repent to Almighty Allah from the sins you have committed, in addition to the promise that you will not commit those sins again as much as possible. If you returned to committing a sin because of the strong desire, or because of the influence of Satan, then do not despair from the mercy of Allah, and repent again, as the Almighty is Merciful to His slaves, and does not close the door of His mercy. Almighty Allah has promised His slaves with His mercy. He said: “Say: O My slaves who have been prodigal to their own hurt! Despair not of the mercy of Allah, Who forgives all sins. Lo! He is the Forgiving, the Merciful.”(53:39).

A person has to discipline himself and strengthen himself little by little by contemplating on the harmful effects of sins in the Hereafter. Repeating this contemplation will strengthen a believer's faith, and he moves away little by little from sins. Every believer who commits a sin should repent to Almighty Allah and seek His forgiveness for leaving one's servitude to Him and being disobedient. Even if one repeats the sin, he should repent immediately, as God Almighty has promised His sinful servants to be pardoned and forgiven if they repent.
Fasting is a helpful means to keep away from sinning and disciplining oneself, and so is the performing of supererogatory prayers (nawafil), especially the Night Prayer (Salat al-Layl).