Hrmm... I guess there may have been some hard feelings for a couple of years, before they "regressed" to the "mean." In the episode, they kind of made it seem like neither one of them had really wanted to get divorced, which is why it struck me as incongruous that she'd have a daughter by another man before their marriage had been legally dissolved.
I mean, god knows that I can testify that it happens sometimes, but I have no practical experience of it happening between two people who didn't have a bad breakup, or continued to be amicable post-divorce.