What should a girl do who loved a young man who had an illicit relationship with her mother, and he refuses to marry her because of this secret, and she cannot give him up?

The young man and the mother must offer sincere repentance for the illicit relationship, by abandoning the sin, regretting it, and resolving not to return to it. It is not permissible for the questioner to marry this young man if his relationship with her mother reached the extent of adultery, because he becomes forbidden to her according to the preponderant opinion of the scholars. If the accusation is mere suspicion, the previous ruling does not apply. It is not permissible for the questioner to mix with a foreign suitor, and she must perform Istikhara (prayer for guidance) and verify the sincerity of the young man's repentance and his commitment to Allah's commands.