When I first read your post of what the ow said in front of your D, my thought was it was inappropriate, yes, no big surprise there. Second thought, why the year 2013? I mean the their R has been going on for quite a while, right?
It may have been something she pulled out randomly to explain why they're not married yet or maybe something XH threw out there to shut her up. Far enough away so he doesn't have to hear about it for a while.
Who knows? One thing I can attest to is that if you sit quietly like Snodderly says, the answers do come. You'll have time to figure out how to handle it with your children with the grace and dignity just like you've handled everything thing else, no matter what happens.