I can only tell you what I do, it's very difficult if one parent is trying to undermime the other, for the other to say well that's wrong will just undermine the other parent. But then again, not to address it in some way maybe gives the impression that somethings are either taboo or simply not important.
OK I found that as my children have grown up how I deal with this has changed.
With my younger DC's circa 10 year old I attempt to address it very quickly and not dwell on it, I try and take it off them and say "We don't agree about something, and as adults it's mum and dad need to sort this out because it's adult things. Don't worry I'll talk this through with your mum and as we'll come to some agreement." I also give alot of support/excuses for their mum such as, "it's very hard because of this ... and or ... that, and she's cross because of this or that."
With the eldest this really isn't good enough, he'll be thinking, what is it your hiding now I'm an adult (well 17), therefore I've found myself discussing more about what IS going on and asking him to try to empathise with both parents and understand us both.
I also have a chat with friends and discuss events to try and get a handle on what they think the mothers motivations and drivers are. The ex just want's things, to feel in control, to lash-out, to feel loved, to blame others ... just going about it the wrong way, I feel sorry for her ... err OK and bloody pissin' annoyed . GRRRRRRRR!
Sorry for babbling on! Ergh I dunno ....