Either she thinks he's innocent, or she refuses to leave him because of her beliefs. I knew several devout women who stayed with husbands who cheated on them and mistreated them.
From what I’ve heard, people such as the Duggars, and Anna in particular here, believe it is the wife’s duty to satisfy her husband, and that divorce is simply unheard of. And the reason is, if a man has strayed, there’s an assumption that the wife did not take care of him in every way. The wife is said to share blame if her husband isn’t faithful.