It's hard to find new friends as an adult, and especially as a new parent. I can make mom-talk with any caregiver, sure, but that doesn't make them a mom friend. Which is why breaking up with a mom friend is so heartbreaking; you finally found a person you connect with and bam. It's over. Watching this special kind of relationship dissolve from the sidelines isn't easy, either, and it can be extremely difficult to know what to say to someone in this situation.
Of course, what to say to a loved one recovering from this specific kind of relationship loss will vary depending on individual situations. If a former mom friend robbed their house before stealing their identity and running off to Argentina, that's going to be very different from, say, slowly realizing the relationship wasn't healthy and needed to end (the latter being the more common narrative in a mom-friendship gone south, of course). Barring the most dramatic of circumstances, it's generally best not to try to make the other person feel better by initiating negative talk of the former friend. You never know what might happen to the relationship down the road and you don't know when your words will come back to haunt you, this may only serve to make your friend feel worse, and negativity, in general, is best avoided when possible.
So if we're not going to talk about how this other mom was the nastiest skank b*tch you have ever met and mark her down as a fugly slut in your own personal Burn Book, how are we going to handle this? Honestly, in many of the same ways you'd help a friend through a romantic break-up. The name of the game isn't "told you so" or "I never liked them" or even necessarily "you're better off." It's all about empathy and giving them space to tell you how they feel.