Anyone can sin, Christian or not. Nobody should sin, Christian or not. Your concern for your wife’s mom & BF should be their salvation. Pray that the eyes of their hearts be open to hear the truth, receive the truth and to choose to become disciples of Christ. Christians should not judge those outside the church. But as Paul advised do what we can (as God directs us) to win as many to Christ as possible by sharing His truth with love.

If your question is merely about what the Bible says about a Christian remarrying after an abusive relationship, I’ve studied this question enough to believe abuse is a valid reason to divorce. There is no direct scripture on this however, so I understand if others disagree. The abuser destroyed what God put together. They are acting like an unbeliever in my view.

I don’t see where God calls us to stay in marital situations where we could be physically harmed. However, Paul continued to serve God when He knew physical harm was possible, so it depends on what your calling is. There are many scriptures that encourage us to persevere through suffering/difficulties. Thus, submitting to God’s direction is always best.