Its not just girls - my DS2 has had (is still having to some extent) pretty much exactly the situation you describe, identical in almost every way except that they are 5 yo boys.
The boy doing this to my DS is clearly a very cleaver child (initially, ironically, I was happy DS2 had such an articulate friend as DS2 has 2 languages and was stronger in English than his school language)... who was massively adoring of DS2 for about 4 months before he "turned nasty".
The other boy tells DS2 he is never right about anything, his drawings are rubbish, his ideas are rubbish, he can only play if he takes some invented humiliating role in the game ... he tells other kids they can't play with him if they play with DS2, that DS2 is a baby... then the next day throws his arm around DS2 and refers to him as his best friend and spends the entire day 1:1 with DS2 and excluding all others...
I have told DS that the other boy is not a friend, a friend would not make you feel sad, and that he should tell him he doesn't want to play with him if he is mean - it is very hard for such a small child to do though (DD had a bit of the same thing when she was 8 and handled it pretty well, following the same advice).
I have also had a meeting with Kindergarten (we are in Germany - Kindergarten til age 6/7) and they surprised me by saying DS2 is not the only boy the other one is doing this to - they already knew about it. They also let slip that they have heard the boy's dad treating him just the same way - bigging the little boy up then knocking him down (Kindergarten aren't quite as professional about not sharing information about other families as a UK school would be!) and DH has said he thought the dad is an arse when he met him (the dad answered the door at the boy's party by saying "we don't buy anything at the door" and left the door open and DH, whom he'd never met before, on the door step, and wandered off... The mum doen't try to hide the fact that she thinks that the sun shines out of her pfb's bottom, but I hadn't realised he had such a nasty, manipulative mind game playing father modelling how to treat people for him...
The girl who is bullying your DD quite likely has a more messed up family than you know, and is quite likely also spreading the bullying attention beyond your DD - school might already be monitoring to a degree. Go in and talk to them (not about the girl's family obviously :o , but about what is happening to your DD).
Kindergarten are also doing a self confidence / sticking up for yourself without violence programme for the kids and are offering it more frequently due to the situation with this boy. They are also going to call the boy's parents in, though I have no idea how that will go...
The other thing I am doing is bigging up DS2'S other friends and inviting them around more often (been a bit slack on that as its harder to do regularly with DC3 than it was with the older kids when they were small, due to schedules of older kids).
Good luck - what ever you do, do not let the school try to make the girls play together - the snide comments and nastiness will just get more subtle and your DD will miss out on building healthy friendships instead - hep her build other friendships by inviting other kids over as you have started doing, do not let her invite this girl over or go to her house, tell your DD to stay well clear, have school keep a sharp ear out for meanness and taunting and put downs etc.