It seemed blatantly obvious that the their earlier leak to People would be integral to the AN case but I do think this wasn't explained properly to M or to her friends
Does it matter? I'd have thought that, if you get involved with media stories about someone who's known to fling lawsuits, there's a pretty good chance of ending up involved in one ... something 5 adult (and presumably intelligent) women might have been expected to work out for themselves
And it doesn't matter whether MM "accepts" her friends will be witnesses or not. They chose to get involved and will have to bear the consequences of that - though if they refuse I guess that's a matter for the court
Either way, I expect what may have seemed a jolly jape at the time now looks rather different