How can one atone for this sin?

It is not permissible for a woman to take a male companion (khalīl/khidn) and establish a relationship with a man who is a stranger to her, whether it be a friendship or otherwise. This is because it transgresses the boundaries set by Allah and violates the sanctity of the husband. Sincere repentance is obligatory for this, by cutting off all contact with that young man, and by being wary of the whispers of Satan that make disobedience appear appealing. One should occupy oneself with obedience to Allah, avoid seclusion and idleness, and keep the company of righteous women.