I would ask your daughter why she thinks the other girl is mean. Help her cope with the situation on her own before getting involved. I encourage my 3 and 4 year olds to communicate and then get adult assistance.
If a child feels the other person is mean, then the child needs to learn to communicate their feellings or preferences of action to the other person... whether the person is a child or adult.
My children tell me things like... brother pushed me... he grabbed my arm... he took my toy... he called me "blank"... he laughed at me... he won't let me play with him... he made faces.
Each of these things children can try to resolve on their own by telling the bully that "you are being mean", "you hurt my feelings when you...blank", "be nice", "be gentle", "share", "it's not nice to call names". Then the child can try to work out a solution.
I try to teach my children to stand up for themselves BY speaking up to the other person. Adults and children often do not fully understand their effects on others, so we need to teach them early on in life. By speaking up and explaining our feelings, I am teaching my child and his playmates how to communicate, the effects of their actions and to have compassion. After something happens, my children are more likely to understand that being mean hurts feelings and they more readily say sorry, give hugs and decide to share on their own.
I still tell my children to let me know what goes on in their day... good and bad. When they express their feelings and the bully continues, I instruct the children to tell an adult. Maybe you can watch the children interact during the playdate and intervene when something happens. Otherwise, tell your child to find the other's parent and indicate that her feelings were hurt when she was pushed or whatever.
You can let the parent know right away about what your daughter said, but I would suggest teaching your child to speak up and learn to cope with difficult situations at the same time. Children and adults often resort to name calling and eventually acting out on the other person because they cannot communicate, find a solution or control their own feelings.