I agree with you, the blame is not to put on feminism, or any other of her ideologies. The problem is that she is offending people who don't agree with her, ruining the times in which you guys hang out together.
First of all try to point out how disrespectful her behavior is, and that there are other more appropriate ways to express her opinion. For example, when your friends announced, I imagine that it was a joyful time, and her comment ruined the atmosphere.
If she realizes that, she might stop being so extreme about her opinions (of course try not to sound rude, otherwise she will be defensive and things will probably get worse). If she eventually doesn't eventually stop, you might consider dropping such a negative person from your circle of friends. Dropping a person is worth keepingit to keep the group together.
EDIT: noticeNote that the opinion of your other friends is important too. I am assuming that almost everyone (if not everyone) doesn't like her behavior.