I actually don't think you should say anything to either of your friends, as it could create some awkwardness or hard feelings between you all. I have a few friends whose significant others/spouses I'm not particularly crazy about but I would never say that to them. My friend loves that person, so for the sake of my friend (and my friendship) the least I can do is make a concerted effort to be cordial and to tolerate that person as well. I certainly won't go out of my way to hang out with them, but if they're around, then what difference does it make if they're there?
Maybe because I'm more of a "go with the flow" kind of person anyway, but so what if this chick wants to take charge? Who cares if she decides what restaurant you all go to or what movie you all go to see? Does anyone else ever have a better idea? Maybe next time she takes charge and tries to dictate what everyone else does, why not just offer a different option, and see if anyone else agrees with you instead?
And the next time she says something negative and mean to you, just fire right back. There are polite, joking ways to put someone like that in their place. And I can speak from experience when I say that I tend to be a REALLY sarcastic bitch...and it just comes so naturally to me that sometimes I say really bitchy stuff to people- it's gotten me in trouble on occasion when I've been around people I don't really know so well.
But you can jokingly say, "Man, why are you so grumpy today?" "Who peed in your Cheerios today?" or my ever-popular favorite, "Oh, well fuck you too, then."
Or next time you happen to be around her (and just her) just say to her "Are you ok? You just seem really down and I feel like you're kind of trying to push us all away....stop being so mean and enjoy the wonderful company, will ya?"
And if that fails, then perhaps you can just tell her she's a raging cunt. But be prepared to be exiled from your group.