Geez! That confuses me too. It would seem that this mother is not one to consider others. Perhaps she's just doing what she always does without consideration that there is a new girl involved. Or perhaps she isn't comfortable being around someone, you, she doesn't know.
I suggest, if you're going to need to interface with her again (play dates?) that you call and perhaps arrange to have coffee with her so that you can get to know each other. Or perhaps you will see her at school and you can say you were confused about the trick or treating. "Am I missing something?" sort of exchange. Mention that all the girls hang out at school.
I'm confused by your post. Are there 3 moms involved and you're taking Mom number 2's kids trick or treating because Mom # 3 cancelled?
After SWH: I understand now. so the daughters of mom #3 are not going trick or treating with your daughter and friend. At least your initial plans worked out sort of. I, too, feel puzzled by mom #3. Sounds to me that she was rude. Also that she doesn't understand how girls interact with each other. How disappointing for all of the girls.