What is the ruling of Islamic law regarding the marriage of a man and a woman who became pregnant by him, knowing that she was married to another man and her divorce was not finalized either Islamically or legally?

The marital bond is only severed by a divorce from the husband or a sharia ruling. The woman mentioned is still a wife. You must distance yourself from her and repent to Allah for committing zina (fornication/adultery), as it is one of the most heinous major sins, as Allah Almighty says: "And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way." (Quran 17:32). The pregnancy is not attributed to you, but rather to her husband, the master of the bed, as the Prophet, peace and blessings be upon him, said: "The child is for the master of the bed, and for the adulterer is the stone," unless the husband disavows it through li'an (oath of condemnation), in which case it is attributed to her mother. If the woman is harmed by her husband, she should go to the sharia courts. If she obtains a divorce and her 'iddah (waiting period) expires, and both of you repent to Allah, then there is no impediment to your lawful marriage.