It's possible she latched onto your son to escape a bad homelife. She may have been molested as a child or endured other types of abuse. Her parents may have neglected her upbringing or she may have had a poor relationship with them, feeling unloved. These are her problems, although they affect everyone in her life. It sounds as though your son is trying to do his best in an unfortunate situation.
People can change over time.
Your son needs your support and encouragement. You might suggest that he find a Christian counselor he can talk to about his homelife. It would give him an outlet for his feelings about this situation, and may provide him with some useful tools to apply to the relationship. Although one person can't always change and improve a marriage, it is sometimes possible to make small changes in attitude and expectations, and in 'handling' the situations that arise, which may result in positive changes. Encourage him to spend as much time as possible with his child, and to go to church with her, even if his wife doesn't go with them.