No one deserves to be unhappy for the rest of her/his life.  Some women get married to men who treat them bad because they've never been with one that treats them well.  If her husband is abusive (verbally or whatever) then she has the right to be with someone who is sensitive to her being. 

And in this case she would be thinking of her son.  She will teach her son that it is not okay to treat women that way.  Just because he is the child's father doesn't mean he is a good role model or a good husband.  I do believe however, that she is responsible for telling her husband that she is in love with someone else because who knows, maybe they can work things out with a little marriage counseling.  That is what I think.