Is there repentance for someone who has committed anal intercourse, and does repeated sin make repentance lose its effectiveness, and does God, by keeping the disobedient person away from Him, give him an opportunity for repentance and closeness to Him, or are both this life and the Hereafter lost? And is this young man who convinced her that this act is not fornication sinful?

You must make sincere repentance and strive against your own self for rectitude. Allah accepts the repentance of whoever repents with sincere repentance, no matter how great the sins, as indicated by the texts of the Qur'an and Sunnah.

Among the helpful means for repentance and rectitude are: increasing acts of obedience, remembrance of Allah, and seeking refuge from Satan; occupying your time with what benefits; avoiding mixing with foreign (unrelated) men; supplicating for chastity and a righteous spouse; increasing in seeking forgiveness; staying away from causes of temptation and sin; seeking righteous companionship; avoiding bad female friends; increasing in fasting; lowering your gaze; and occupying your mind with beneficial knowledge.

You should remember the warning against indecencies and the clarification of their consequences. You should also read books that encourage repentance, clarify the importance of modesty before Allah and being mindful of Him, and recall the terrors of the Hereafter.

You must strive for rectitude and not despair if you fall into sin, and promptly repent again.

Anal sex is forbidden and is considered adultery (zina) which necessitates the prescribed punishment (hadd) or discretionary punishment (ta'zir). Both the front and back private parts are areas that must be covered, and there is no difference between them regarding the necessity of ghusl (ritual bath) upon penetration.

You must preserve the obligatory prayers and voluntary prayers, for they forbid indecency and evil. And increase in the remembrance of Allah, for it is a fortress against Satan.