What do you advise concerning a woman who left her husband because of his adultery, but did not divorce him, and later lived in adultery herself?

Question:

Question: I have a bizarre situation.

I met a woman, not a member of the church of Christ, who married an atheist when she was young at the urging of her grandmother to avoid fornication. However, she was not tutored properly about Christ’s teachings on marriage.

Some years down the line she became privy to her spouse's extramarital affair with another woman, which was verified. She desired to divorce but remained in the marriage due to insurance reasons. Unfortunately, relations between the two eventually deteriorated until they parted ways without completing civil divorce proceedings.

The woman traveled to another and eventually forges a new relationship with another man. She filed for divorce in her late twenties from her spouse because she had become pregnant. The law in the state she moved to said the child gets the name of her current husband regardless of who fathered the child. Her new partner wanted the child to bear his name. They cohabitated for about five years but never marry. This relationship then failed.

She believes she can marry because her husband severed their marriage covenant first. I advised her that Christ has some stringent rules about remarriage but never speaks on that. Can you share any input? Are there are any Scriptures or scenarios I can use to illustrate if she can remarry now or should she try to return to her spouse, who is now remarried, or remain celibate until death if she desires to please God?