5 year old with bullying friend

My daughter is in kindergarten.  She just turned 5.  She has a very unhealthy relationship with one of the girls in her grade.  They were in the same class last year, and this year, they are in different classes, but share the same lunch and recess times.  This girl has my daughter under her thumb.  She constantly manipulates her.  At a party last week, she told my daughter she had too much candy from the pinata and attempted to get my girl to hand over her candy so she could take it.  I stepped in and told my child I would hold her candy for her while she played.  Later, the girl got angry with my daughter when she didn't hear her calling in a crowded room.  She told my child "you have to listen when I'm talking to you."  She physically restrains my child from playing with others at recess.  Tells her she has to wait for her on the playground.  And always picks the game and the other girls who will play.  I have tried to convince my daughter that she needs to develop friendships with other girls.  That other girls see that she is best friends with this girl, and that they might be put off of being friends with her if she only plays with them when she isn't getting along with this girl.  The school is aware of the situation, and they helped by putting them in separate classes, but they can't keep them apart on the playground.  My daughter is like the moth to the flame with this girl.  We read "My Secret Bully" and "Simon's Hook" but I can't get her to play with someone else.  For a while, she was making friends with a new girl in her class.  But now it seems that the bully has usurped that friendship, and is excluding my child from playing with the new friend.  I have tried to encourage other friendships through play dates with more suitable girls, but I can't wean her from the attraction for this girl.  Does anyone have any suggestions?  My little one is coming home with constant stomach aches from these interactions.