Most Helpful Guy
I'd imagine she still does feel some sort of connection to the boyfriend. And doesn't' like the idea that the father of her child is seeing someone else. But I think this is something that will fade particularly if you two just kind of let it fade to black. If two of you just sort of appease her for a bit, kill her with kindness but also don't acknowledge the nasty crap she says perhaps she will realize that her actions aren't getting the reactions she's hoping for.
most often when people are acting childish/petty/etc. what they are really feeding of is the reactions they get. So a child stomps around the house when they don't get what they want because they hope to get attention that will re-enforce their behavior (stomping=attention). I'd try to not re-enforce her actions or at least not re-enforce them the way she wants. Additionally I'd continue to maintain a relationship wiht her new partner maybe even forwarding specifically nasty stuff (if appropriate for him to read) so that the new partner could address it with her directly, cause frankly I'd be mad at my wife/partner for behaving the way she is