Actually, that depends. IF this mare is an issue and is endangering other horses or owners, it is very much the BO's business. After all, it is her liability if something happens. SHe has every right to tell the horse owner to fix it or leave. My guess is she may have told this owner to fix it, and that has not happened.
I am not thinking we are getting the whole story...something is just not right.
The whole story, I believe, is yet to be told.