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Your husband may want to have a discussion with the ex wife about what is her purpose for continually feeding the daughter with such untruthful nonsense. He may need to tell the ex wife that she is virtually doing more harm than good.
Your step daughter has already heard one side of what happened in the fight. If she hears another side, she will only be further confused and really not know who to believe. Your step daughter does not want to have to choose sides but her mother is putting her in that position. You nor your husband should want to be the "bad guy" to your step daughter. With regards XXXXX XXXXX fight, do not bring the issue up unless she mentions it...which she probably will not. But in the event that she does, simply tell her that there was a grown up discussion and one person felt one way and the other person felt another. That is basically all that needs to be said.