I find these situations incredibly difficult, too. I usually try not to say anything. But seeing that she criticised your parenting style by saying: "You're just a first time mum and you will get it once you have a second child", I think I would not have been able to hold back after that. I can get pretty defensive about my parenting style. Yes, it is hard sometimes (going through teething at the moment, too, for what seems like forever), but I think it is a small price to pay for my daughter's long term emotional development.
I agree with what some people have said that we should hand out free copies of books like "The science of Parenting". If people got more than one kind of advice in the early days, they might be more open to gentler options.

I have been told by an acquaintance (who doesn't have any children of her own) that had I not moved away from all my friends in Sydney, I would have a much "healthier" relationship with DD. And then she told me about another acquaintances "healthy" relationship with her DD. She has been back at work full time since her DD was 3.5 months old, She goes out at least one night a week without her DD, only breastfed for a little while, sings the praises of the convenience of a planned C/S, does not only CC but actuall CIO, etc.

When I get defensive, I usually tell poeple that being a parent is not about convenience. That having a child is one of the most inconvenient things one can do. But that the rewards are just so great, that we don't seem to mind. And that we all do what we think is best for our children and I happen to think that letting them CIO is bad for them.

My FIL has told me on many occasions how they would let DH CIO for hours on end in the first year of life. How that was horrible, but necessary. And then he wonders why DH has troubles talking to him when he has a problem. Why DH doesn't seem to feel he can go to his father for help (his mum died when he was 9). Why he keeps his emotions locked up inside. Ok, lots of guys are like that. But maybe that is because they were told to tough it out when they were little. He also told me lots of time: "You should get her on the bottle (since she was about 3 months). She'll be a better kid for it. And you'll have your life back. I never bothered to ask him how she'll be a better kid and how I'll get my life back while I'm in the kitchen washing and sterilising bottles and spending money on formula...

I have a problem at the moment because I have a new nephew (less than 2 weeks) and I can see so many thngs his mum does that I would do differently (like giving him a dummy from day 1, spacing out his feeds, only BFing him once a night, etc. But I have to let them do it their way if I want to be a part of his life. I don't want them to resent me for being condescending (is that the right word). I'm also worried that my criticism might make them more stubborn about doing things their way. I want them to feel free to aks me for advice, should they ever need it. I am however very happy, that they let him co-sleep at night. I'm especially happy because I know my brother is infatuated with his son, and this way he gets lots of extra cuddles when he comes homw from work. Actually, it sounds like he has bonded much better with the boy than the mother has....

Anyway, enough of a rant,...
I hope you can find a way to remain friends with these people if that is what you want. Maybe one day, when the emotion of this situation is not high, take her aside and just say to her: "Look, I have tried hard - against my instincts, not to judge you and criticise you for your parenting choices and I would appreciate it if you showed me the same respect because we all do what we think is the best for our children and ourselves, based on the information we have at hand" I would offer her to lend her some books if she wants to understand why you do things the way you do (not to change her mind, but to understand your perspective).
Maybe approaching it from such a dry view point will let her see that she was unfair to you. But I don't think anything you can say will make her change her mind.

Good luck with everything.
Sasa