@Lash,
My experience is from the jealousy side of things -- I was in a new relationship with a guy who had been in a serious relationship with someone before me. This woman had a small child, and he had a relationship (though he was not really a father figure) with the child, too. They were together for about two years, and much of it was long-distance, so the scale is different.
But I absolutely hated the thought of them spending any time together, while also realizing that was my problem. He was understanding of my jealousy issues -- he had parallel jealousy issues with
my ex, which made things easier for both of us. ("Thought experiment -- does Current actually have anything to worry about when I see Ex? No. So I should chill out already when he sees HIS Ex.")
But both of us operated from the understanding that the jealousy issues were silly and something for us to overcome, not something that justified dictating terms of contact with the respective exes.
And here's the thing... we both got over it. After much teeth-gnashing early on, we adjusted, and the old relationships faded naturally. We do still each have the occasional contact with the exes (the two of us are still together, 18 years on), but it's just not a problem anymore.
I do think there are situations when people stay enmeshed with their exes and are unable to move on emotionally, which is a bad thing. I don't see a lot of evidence of enmeshing here, though. (Perhaps I'm wrong, and if so, my advice changes.) But as others have said, one can have a cordial relationship without that constituting enmeshment.