I'm guessing the FB group is for something like PTSD, or narcissist victims or similar. I am in such a group and it is laughable how often the posts are themselves transparently narcissistic/toxic. People often can't see the mote in their own eye, myself included.

Regardless your concern seems less with what happened to your friendship and more about exerting control / influence over the life of your toxic ex-friend. Walk away.

I see you know my ex-wife.

When I deal with toxic people, I remove myself. Other adults can choose if and when to remove themselves from a situation. Whenever this happens, I find out others get burned by the toxic person. The people dealing with toxic person now will probably be running for the hills in six months.

So while we were friends, this ex friend continually bashed others behind their backs and unfriended people when they did something she didn’t like.

We’ll now it’s happened to me. Hooray for me, actually.

But the point of this post is that she and another woman I know lead a FB support group for a certain condition that I am a part of (that’s how I met her). I really want to tell this other person about how toxic this ex friend is and how she shouldn’t be letting this woman run the group anymore.

FWIW, the woman DOES know she is toxic because she’s had to referee fights btw her and other members of the group.

This toxic woman should NOT be helping to run a group that has vulnerable members.