Could you not respond in any way, shape or form to her rants or conversation about her child except with a general 'Huh.'? Basically, I guess you could say don't give her any positive reinforcement (i.e. a conversation) on ranty/kid-centered topics?
If she's been a good friend and you think you can get away with it, what about a somewhat light-hearted but blunt clue X 4 type of comment?
Her: (Rant about bad parents)
You: Huh. (pause) So have you met any good parents lately, or is everyone else a bad parent?
Her: (Rant about bad parents)
You: That's HORRIBLE! Gosh, how do they live with themselves? Boy, you should call the authorities! Imagine, putting their kids in a licensed daycare in order to provide a better income for their families!
or...
Her: (Rant about bad, working parents)
You: Huh. Well, I guess you don't want to associate with me anymore.
Her: What? Why?
You: Because remember? I worked when my kids were little too. When you rant about how bad working parents are, you're basically saying I'm a bad parent. That doesn't sit so well with me.
Her: I wasn't talking about YOU!
You: You might as well be. Nobody's perfect. We all make different choices, and we all do the best we can. Different choices are not necessarily wrong, just different. So can you cut it out with the rants? (you could add, 'it makes you come across as really judgmental and holier than thou, even though I'm sure you don't mean to portray yourself like that).
or...
Let her get in a few minutes of talking about her kid, but then after that time, don't respond directly to ANYTHING she says about her kid. Nothing. Throw in a totally unrelated comment. If she says anything, you can say, "Friend, I know you love your child. That's great. But there ARE other topics of conversation. (Don't pause). Did you hear about that new little sandwich place that opened up?"
Call it to her attention, redirect her, and don't give her a choice. Then if she's embarrassed, it gives her an out to go on to something else without dwelling on it further. And if she wants to dwell on the kid thing, you can say, "Do you realize that since this conversation began, I've heard A, B, C, D and E about Child. Seriously, that's not a conversation, that's a blog. It's not that I never want to hear about her, but come on, not a 15 minute monologue containing information about her pooping habits, okay? Now, did I tell you about that cute jacket that I found on sale?....."
Please let us know if anything works, or how this plays out. We all know people like that.