Hi, there. As the mother of a preschooler, this hurts my heart! I approach this situation from two different perspectives, to be honest--a mother and a teacher. As a teacher, I would really desire to know that something like this was going on in my class, especially if it involved little ones. I would recommend that you gently advise your friend to advocate for her child. She needs to speak to the teacher about what is happening. Chances are, the teacher has no idea that the "bullying" is going on. I feel strongly that if she did know, she would be more observant and look for opportunities to talk to the children about being kind to one another. Sometimes that is all it takes. Another thing the mother might do is ask for an opportunity to visit the class and do story time, or bring a treat, or use her daughter as an assistant, or do something else that by association will make her little girl "the cool one"--the classmate that everyone wants to play with.
If these do not solve the problem, the mother definitely needs to speak to the preschool director about the situation. The little girl is far to young to have her academic experience tarnished by something like this.
In considering my response, I think the mother can also talk to the teacher about bringing her daughter forward as "the cool one" by using her as an assistant, praising her, etc. This does not have to just be the mother's job. :)