How to handle friends son that has turned into a bully?

Some very good friends of our have a son who is in my daughters cl at school. They use to be best friends. Now he has turned on her and has even gone to the extreme of telling everyone in there cl not to play with her. (they are in kindergarten) I'm not saying my kid is perfect but she is a a shy quiet little girl. I have never seen her or been told that she is mean or agressive. I talked to my friend about this and he got in trouble and gave a obviously forced apology in school today. When a girl went to play with her during recess he then turned the cl against this other girl because she was playing with my daughter. I hate seeing my daughter cry that she doesnt want to go to school and come home crying that no one likes her now. The little boy has apparently become popular in the kindergarten social scene and is doing this because my daughter isnt as popular because she is quiet. I dont know what to do. I dont want to constantly be calling my friend pulling the whole "your son was mean to my daughter again" but this is really affecting my daughter and our friendship with our friends. I dont feel like even hanging out with them anymore because then my daughter will be around him and play and act fine out of school just to hurt her the next day and be a bully at school. Help I dont know how to handle this??? I dont want to loose our close friends but I dont want my daughter being treated like this. i cant stand her being upset.