I kind of think you should send an email to the other members. Just a heads-up without gossipy details so they are not caught off guard if this woman does call them.
Say something like "Just an FYI, Mrs. X is no longer part of the play group." That's just an idea. I'm sure you or other posters have better wording.
I agree with this. If you don't, there is a risk she could ring one of the other mothers to find out your next location and show up to make a scene. If there are multiple names in a CC on an email, I wouldn't notice one was missing or assume it was on purpose. Equally, in an online group (I'm not sure exactly how your system works), I wouldn't notice one member having gone.
The point of the above is that if I knew someone was part of the group (from having seen them at a meeting) and hadn't been told that was no longer the case, I'd see nothing wrong with giving the information to someone who 'accidently erased the email' or 'was just calling to check the location'.
Edited to remove incorrect apostrophe.