If she then, after being sent away, marries another man, then she is also guilty of being unfaithful.” In those days men would sometimes send their wives away for any reason without giving them divorce papers, leaving them destitute and unable to properly remarry.
So if she gives herself to another man while her husband lives, she will be guilty of unfaithfulness to her husband. But if her husband is no longer alive, she would not be guilty of unfaithfulness. His death has released her from the law of marriage.