Wow! I'm not going to try to speak to the past, the situation which led up to the step-daughter leaving, as that is in the past now...

What matters now is really how things will progress in their relationship going forward. It sounds to me like the stepdaughter will "virtuously" tell everyone that her step-mom went off her rocker in her presence, and even thought she "did nothing to cause it," she was kicked out.

Meanwhile the step-mom is feeling proud of herself for finally taking action, and no longer feeling like a doormat...

The challenge is going to show up when the time comes -- and it will -- that the two of them need to communicate with each other or be in each other's presence for some reason. Could be a family illness, could be a family event such as a death or a wedding.

The fact is, IMHO they were both wrong in their actions and reactions. I predict there will be "blood," hypothetically speaking, before things finally get ironed out.

I suspect that the only way to avert this is for the step-mom to call the step-daughter now, and "eat a little crow" by apologizing for her "bizarre" actions --even though I am sure she feels no need to do this, because in her mind she is in the right -- and invite the step daughter for a chat so they can "iron things out now, before things fester and get worse between them."

They ought to meet in a neutral zone -- such as a restaurant or park or other public space where they can talk -- and the step-mom needs to tell the step-daughter how she feels taken advantage of. And then use the "detente" as an opportunity for a "teaching moment," where she explains to the step-daughter how proper house guests behave.

I'd do it now, so that peace can reign in the family and the hurts on each side don't have a chance to grow larger fueled by time and distance.

Not sure others will share my feelings, but that's my two cents worth, gained through growing up in a periodically dysfunctional family. (Didn't we all, to some degree?)