This is a little harsh, don't you think?
Are we all supposed to just stay with who we're with because, what, we know their quirks and shortcomings? We are all human, capable of mistakes, capable of changing and warping into different people. The person you started your relationship with may not be the same person you end up with.
Yes, if she's cheating on her husband, whether online or otherwise, she's going to have to live with that decision. But that doesn't mean she should stay with the person she's married to if she's no longer happy with him (as he's probably not very happy either), purely out of obligation or a fear that the person she leaves him for may down-the-road, leave her. We should stay with people, work on relationships/marriages, if we still truly love the person we're with and want to work through our problems rather than mask them. But I can tell you from experience that staying with someone out of obligation, guilt, or fear, is not going to end up doing anything but causing more pain.