@aSofaNearYou
I think that the when the impact from bullying is significant, it is so unimaginably painful that it is never something that should be treated lightly.
As you suggest, I am a big proponent of both self reliance and knowing your lane.
I don't think that the OP had any obligation to put her daughter's behavior on radio blast.
I do think that the OP's partner had an obligation to alert his daughter's mother to the situation, and I think that the mother had s right to be concerned.
The OP 's daughter to make mistakes is in no way curtailed by the 9 year old's mother wanting her moved to a different room. The 9 year old's mom is not trying to tell the OP how to parent or manage her household. Her only request relates to the placement of her child.
The 9 year old's mother is not broadcasting the 12 year old's business nor is she publicly or privately vilifying the 12 year old. The mom is not even asking that the 9 year old be removed from the house. She has simply said the 9 year old and 12_year old roommate placement may be working to the detriment of her child.
That is neither a vicious or extreme response. In my opinion, it is a tempered response to a regrettable situation. Had the OP's partner alerted the mom to the situation and explained his plan for handling the situation, the mom might have felt that the dad was handling the situation in an appropriate manner.
Instead, the mom was left to be informed by the 9 year old, and none of know how much or how little the situation may have impacted the 9 year old.
Again, what parent does not take an interest in the kind of company that their 9 year olds keep. If your 9 year old had a friend who on a lark or in keeping with his character decided to shop lift in a candy store, I think that we would all agree that this was hardly a serious crime and more likely a childish prank. However, many of us might have reservations and concerns about our child being alone in a store in the company of said child. We would also be much more attentive to the interactions that our child would have with this child.
The mother is simply being attentive and trying to put safeguards in place. We say that the OP is doing the right thing by being attentive and putting proper safeguards in place with her daughter, but we castigate the 9 year old's mom for wanting to do the same.
I would imagine that the OP has alerted her daughter to avoid some of the girls involved in this incident.