In my view, she made her choice when she moved out on account of meeting someone new. If my partner did something like that, I would feel disposable. I doubt I would be able to trust them with my emotional well-being again.
nyc is 100% correct that getting your own girlfriend won't solve a thing. Not only the way it treats the other person, but it just won't work anyway. Suppose you and the new girlfriend break-up right when your wife and her boyfriend are getting really serious. What then? Will you expect her to drop everything until you're hooked-up again? Or, more likely, will you feel doubly lonely because you not only lost your girlfriend, but your wife is emotionally unavailable too?
Poly works best for most people when each relationship would be capable of standing on its own. While it's true that "a la carte" dating works for some people, most find that a relationship which meets only some of their needs tends to feel fulfilling. I suppose it would help to know what ways she's feeling unfulfilled and whether those are the kinds of things that can really be "substituted" by someone else. e.g., anal sex and going camping can often be substituted; intimacy and compassion usually cannot.
As I see it, the fact that your marriage broke down once over this is telling. She did not go into this thinking "I like this new guy, but my husband and my family are more important than someone I just met."
Fool me once, shame on you.....
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker