The author of this article feels that parents need to go to bat for their ex, in order to help them be a better parent to their children:
I have learned one of the very difficult lessons of divorce: a divorced parent's responsibilities include not only being the best parent, but keeping the other parent in the loop. This translates into helping your ex be the best parent he or she can be -- even though, especially in the beginning of my divorce, I'd have preferred if my ex had moved to Mars.
Read the full article at: http://www.huffingtonpost.com/judith-ruskay-rabinor/going-to-bat-for-your-ex-_b_2645471.html?ref=topbar
Do you think it's possible to befriend the ex, and in extension his or her new partner? Do you think it's your responsibility to help them be the best parents they can be to your child(ren)?