Agree on Highlighted part ? Share your opinion in the comments

Well, it's a basic falsehood. There are always other options. The more pertinent question would be whether cheating is justified in this scenario and I would say that depends on context. If she cannot easily leave the husband because of the legitimate fear of violence then I would suggest that she would be justified in cheating. If his behaviour broke all the fundamental assurances of a relationship and he were physically preventing her from leaving, or was abusing her in an equivalent manner, then it's a relationship in name only. In that respect she would owe him nothing in particular and that includes loyalty. You could argue that a woman, even under the threat of violence, could seek refuge with family or in a shelter but those options are not always available.

If it were merely an unsatisfying relationship and she was not in any way under any kind of threat or facing the equivalent level of abuse of any kind then cheating is not justified. Even if she couldn't leave immediately she could be honest with him about the relationship being over.