Personally, I have absolutely no problem in understanding how to discuss her methods without getting libellous. You say that you think the method is, for instance, ineffective or damaging to the child or whatever. And you refrain from imputing motives to her of which you know nothing, and making direct personal attacks. Easy peasy. Except that many people simply cannot control themselves.
However, that is really by the by. The real issue is this dispute NOW. As tempting as it is to get all pugnacious and so on, can I point out that it seems to me that GF's decision MUST be highly emotional. As it has been pointed out on here, it will do great damage to her if she were to pursue it. And the fact that (as we all agree) she has made this situation worse and not simply shrugged off what was, admittedly, a long stream of puerile and vitriolic abuse only goes to show (IMHO) that behind this is a person who is genuinely hurt. But I'm not surprised at all that she has reacted this way. I don't think that by publishing a book on sleep and feeding patterns, one necessarily expects to be up for attacks like, say, Tony Blair or something.
Anyway, a good lawyer never EVER advises their client to litigate. Certainly not in a case like this. So, like Anchovy, I am skeptical that this will end up in court. However, the best way to ensure it would do so would be to up the ante in the fight, so that it might be driven at her end by emotion and not pragmatism. Clients frequently ignore their lawyers' advice because they feel strongly about it, even if it's not the wisest course of action. The more we react on here, the more we drive her into a corner where she is angry, hurt and can't back down without losing face. Of course she is seeking damages for what has been said about her. She's feeling very bruised and battered. And this place simply hasn't let up on her - and I don't mean just since this thing erupted. I've been on this site longer than most and it has made regular appearances as long as I've been here, with the nastiness only gathering intensity over time.
I really think the best thing for MN is if we all go quietly away and leave it. They know we support them and they know that we will do what we can to keep MN above water. But I am sympathetic to both sides and think it's really really sad and awful that it has come to this. And frankly, I think the lack of self-restraint (as opposed to self-censorship) from some of US has caused this problem for Justine, Carrie, Rachel et al. Exercising some now would be the kindest thing we could do.