Thank you for your question. When another child is born into a family it is very important to include the other child. Even if it is simple things like including in talking care of the child.
But if the child feels excluded they tend to lash out tot he person they care about the most and the reason why is because they feel that person will understand.
But you can not have your daughter getting hurt by this person.
Your daughter is very use to playing with that girl she once knew and to your daughter she just only remembers the girl being nice too her. She will not be able to understand why this person is no longer nice.
Your daughter will also give her many chances to change because she has always been her friend.
Some times people as they get older out grown their friends, but I feel this is just a child that is not getting attention from her mom and is acting out in a negative way.
The reason why they act out is to get attention, a child does not care if it is good attention or negative attention.
They just learn what every way they can to get attention.
This is not good because this negative actions will continue unless corrected.
I feel like you should point this out to the mother until she realizes things need to change.
Your daughter will in time make new friends, but children only seem to connect with certain children.
They are very young and do not understand when someone is mean. They have always been friends and have that bond as friends for a long time.
But things have changed.
Your daughter will continue to want to see her until she realizes that child is not being very nice. But your daughter will find a friend that she lies to be with.
She just has not found that person yet. But keep trying.
It is important for her to be social and find other friends since the other child is not being the person she once was.
You want your daughter to have a friend that is kind just like she is.