I just had a couple questions that will help me to answer your question.
Does your child see the other young lady outside of school or just during the school day?
Are there any changes going on at home that could be causeing your daughter to revert in behavior at all?
I think this is a difficult situation for you and is stressful as well. I would suggest that for now you continue to support your daughter and teach her about friendships as you have.
I know it can be very difficult to understand her reverting back to "baby" talk but I do not think that this has much to do with hanging out with the friend.
Could she be afraid that the Nanny will be moving on once the baby is born?
A lot of times children will "act" a certain way when they are trying to make sure they will not be replaced (example acting baby-ish when a new baby is coming into the picture). This is very typical development for your children. They have a hard time expressing emotions. Instead of expressing emotions like adults children tend to act out their emotions.
I would for sure talk to her more about the nanny change. This is most likely the reason for the change in behavior you see. I am not saying you should ignore the behavior at school but more so know why it is likely happening. I would suggest a nice talk during a quiet time about what the teacher has told you. I would explain how the exclusiveness and finger pointing and such really hurts other childrens feelings. I would also talk to her about how she would feel if another child was acting this way to her.
The good thing is that this too shall pass. As a parent we tend to worry so much about our children that it can seem insurmountable at the time. Keep in mind that this is a phase (I know easier said then done) and that soon she will move onto something else just as worrisome.
I know as a parent myself sometimes I get myself so upset over something and in a few days, weeks, or a month or two the issue has totally passed. Hang in there and if I can be of any other assistance please don't hesitate to ask.
I hope I have answered your question. Is there anything else I can help with today?