First of all, men lie. There's a good chance she's under the impression he's not married or the marriage/relationship is over.
Secondly, presumably the other woman doesn't even know the wife/gf. Why is the onus on her to respect the marriage/relationship when the man is clearly choosing not to?
The man made vows or commitment to another woman and the other woman did not. The only one in breach of those is the man but it's usually the woman who is blamed.
If a guy is going to cheat, he's going to cheat; it doesn't matter who the woman is.
Obviously if the two women are friends it's completely wrong...
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The thing is that a lot of mistresses know that the man has a wife/gf. Not every time obviously, but I'd say even then, I think they are kind of stupid for it. They kind of avoid seeing the truth.
Anyway, when they know, they should be blamed just as much as the husband. Helping someone to cheat is just as bad as cheating itself. It's being an accomplice. It's funny how some people rationalize it like they are doing nothing wrong when they clearly are. "They are just going to do it anyway" is not a good excuse at all.