First of all, "if a woman has a husband who is not a believer and he is willing to live with her, she must not divorce him" (1 Corinthians 7:13). So Cassie would probably be expected to stay with him despite how he abuses her. In fact another passage we read on the same day from Ephesians basically says that women should obey their husbands and children their parents, so whatever Alex says, should go. So long as he's the man of the house, he's in charge. If Cassie were a Christian, she would not have an easy out. She would have to decide if staying with him is abiding by the Bible and the expectations of God, or is too dangerous, and something that God would never want for her and her child. This is one of the passages that gives us the idea that divorce is never acceptable.
In addition, "a wife must not separate from her husband. But if she does, she must remain unmarried or else be reconciled to her husband" (1 Corinthians 7:10). So even after, if she does separate from him, she must never remarry. In our modern day, this would be considered harder than dealing with cancer or a death in the family. Simply because we believe that a significant other brings value and worth to our life. We think that our life means something only when there is someone to share it, and never being able to remarry would be hard for us to conceive of. I think Cassie would be all right with this because of her past with Connor, and how she's never expected to be loved the way Alex loves her. I think she would/does believe that she has been blessed to have been loved, and she has received more than she deserved. So I don't think she would be distraught by the notion of never remarrying. But any normal girl suffering through the trauma that she has would be utterly dismayed to loose their best friend as a child, and then be abused by their husband, and not have the opportunity to find happiness later.
And lastly, this passage reflects that a man and woman who are married are to give themselves fully to the other, body and soul. "The wife's body does not belong to her alone but also to her husband. [and vice versa]" (1 Corinthians 7:4). How does this tie in with his abuse of her? And her pregnancy? I believe that she has abided by this rule or expectation and been completely vulnerable and completely there for him in all of his pain and suffering, but in no way was he there for her. She has basically been a punching bag for their entire relationship. And she believes that she is made for this role. To be there for him in every kind of need that he has. But since they are married, there is more expected from his relationship toward her. He should be expected to love her completely and respect her body as if it was his own, and never hurt her. For her pregnancy, I believe the same rule applies. Although he never wanted to get pregnant or have a child, since it has happened, he is expected to treat her body as if it was his, and love her completely. He should be expected to treat his wife and child with respect, love and care. And I don't believe he is doing any of these things by the way he is acting.
Abuse within marriage is a hard topic, especially when talking about the boundaries of Christian marriage. But I believe that Alex has never treated Cassie the way the Bible expects a wife to be treated, and therefore, she is free from any of the bounds of marriage. Although she's not Christian, she has given herself completely to him to help him, and it hasn't gotten her anything but hurt. I believe a person like her deserves a love greater than she's ever experienced. I hope if she were a real person, she would find the love of Christ someday. But since this is fiction, I have to believe that eventually, she'll find peace.