I guess it's possible this guy just isn't a good personality match for your wife, and he's now realized that. Might not be a matter of her "not being good enough." I hope not, anyway.
Re: your 8-year-old ... kids are often surprisingly perceptive; most poly people I've observed have had to say something to their kids sooner or later. Something to just mull over for the moment I guess.
Sorry you find yourself in this awkward situation. You are doing the right thing by refraining from "speaking your mind about this guy" to your wife. She's still hurting from the break-up, and will be for awhile. She'll need your ear to listen on, your shoulder to cry on, and perhaps some personal space, who knows. The time to say, "Eh, I didn't like that guy anyway," will come at a later time.
Think of it as an opportunity ... a chance to demonstrate to your wife that being with *you* is still a good and certain thing. Just be there for her, you never know what kind of support she'll need.
One of the "weird" things you're faced with in the polyamorous world is when your parter breaks up with *their* partner (who isn't you). The strangeness will wear off; just try and concentrate on empathizing with her, as best you can.
Love means never having to say, "Put down that meat cleaver!"