"Yup, I wish them good luck, but that's not going to stop me from sharing the fact that I think it won't work."
YOUR OPINION IS NOT A FACT. The harsh, judgemental language which is threaded throughout your "advice" does not label you as a friend, or even someone who likes the person to whom you're trying to giving advice.
I recognize the absurdity of bringing this up on the threads of a sex fetish website, but sex is only one part of a satisfying relationship. It's an important part, but it's apparent from what the woman wrote that she's quite satisfied with her relationship with her husband in many other aspects.
Your idea that they're not trying to satisfy each other is just silly. The woman and her husband do not have the same sex drive. They're talking about it (an accomplishment in itself, since the subject is so uncomfortable for most), and are looking to executing a solution which they both buy into.
My wife and I were intrigued by the idea of an open relationship. When we had affairs with others it didn't help our relationship, and our feelings and actions have changed. I make no presumption based on that experience that various forms of an open relationship will never work for anyone.
It's interesting to see what other people are able to work out in their romantic relationships when they employ open communication.