I make of it that she isn't an inhuman monster, and as I've said several times on this thread, sleeping with a married person isn't honorable and you shouldn't do it. My first post to the OP said "don't do it again".
At the same time, though, while of course it's better not to participate in it, his marriage is ultimately on him and the buck stops with him. She's irrelevant, it's only when he decides he's going to stick his dick where it doesn't belong that betrayal has taken place.
If I am not responsible for my promises, who the fuck is?
If entering a marriage doesn't make me more responsible than some total stranger, what's the point?
Suppose you do scare off the OW and blame her for everything, but your husband is still a sexually incontinent tail chaser - what then? Great, you've blamed her. Now when he does it again, he can tell you it wasn't solely his fault, there just shouldn't be any available women in the world to make it possible!
I know I'm totally against the tide here. I know I can't convince MN at large that a married man is totally responsible for where he sticks his dick. And I know that by saying it, people will accuse me of being an OW myself (no) because the need to blame women for this is so deeply ingrained, they can't imagine any other way to reach this conclusion.
But it's true. And the flipside that the detractors never see is that this applies just as much to me. If I cheated on my husband, I would not dream of telling him he should blame OM, that it wasn't entirely on me, that other men should just never be available. I couldn't begin to try such a flimsy, weak excuse. My fidelity is on me.
And that's a huge reason why I keep it.