What should the questioner do to atone for her sin and for God to forgive her for what she caused the young man, and will the young man know on the Day of Judgment that she was the cause?

Relationships between non-mahram men and women are forbidden, and whoever engages in them must immediately repent to Allah. Ending the relationship with this man is the right thing to do, and the intention must be renewed for the repentance to be purely for Allah's sake. Atonement for sin after sincere repentance is achieved through righteous deeds such as prayer, fasting, and charity, in fulfillment of Allah's saying: "Indeed, good deeds do away with evil deeds." The young man has no rights over you, and beware of the whisperings of Satan that may tempt you to enter into forbidden relationships again. The knowledge of sins on the Day of Judgment rests with Allah, but Allah conceals the one who repents and whose repentance is good on the Day of Judgment, as stated in the Hadith: "I concealed them for you in the world, and I forgive them for you today."