Is she sinful, and what should she do, after stealing her mother’s gold and giving it to someone with whom she had a relationship when she was 17 years old, despite her repentance, seeking forgiveness, and regret?

There is no sin upon you if you were ignorant of the impermissibility of what you did, for sincere repentance expiates all sins. However, the guarantee of the gold owed to your mother does not lapse with repentance; therefore, you must either seek her forgiveness or guarantee her gold for her.