Woah woah woah. She just started getting to know this guy within the last month, she just started talking to you about this within the last week, and suddenly it's poly, D/s and this particular guy, right now, or she's done? That may be honest and open, but I sure as hell don't see how it's supportive.
In your reading, perhaps you've come across the ironic phrase "relationship broken, add more people"? If you guys are going through such a serious rough patch, polyamory will make things harder, not easier. Poly should ideally happen only when your relationship is as strong as possible. So, if you've been together 9 years and this is the low point, this is the exact wrong time to open up. If you both really want to make things work, you'd do better to take a while... months, maybe a year... and solve the problems you're having, through therapy if necessary, THEN consider bringing in another person.
On the other hand, it sounds like she may not be giving you a choice. Does her writer friend know how blindsided, hesitant and hurting you are? If not, for the sake of honesty he should. And if so, and he's still prepared to go forward with it anyway, that doesn't show the level of respect for the existing relationship that you want in a metamour...
It's not that this can't possibly, under any circumstances work, it's just that she's setting things off to a very very shaky start, and she should reeeeeally considered slowing down if she wants to keep her marriage with you intact.
What do you mean when you say you know the root cause of the unhappiness is your own fault?
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.