Is marrying a woman with whom one had an illicit relationship, despite the mother's disapproval, preferable to leaving her out of concern for not wronging her?

Not marrying the woman is not an injustice to her; rather, she is the one who wronged herself by establishing a forbidden relationship. Your duty is to repent from the relationship by abandoning it, regretting it, and resolving not to return to it, and by increasing good deeds and concealing what happened. Marriage to her is not a condition for repentance; in fact, it is impermissible to marry her if she has not repented a sound repentance. You must obey your mother in not marrying her, even if she repents, for obedience to parents takes precedence. We advise you to hasten to marry a religious woman. If you are unable to do so, then you must fast, guard your hearing and sight, keep good company, and make abundant supplications.