Right, I shall try not to drip feed but I am still trying to get my head around the situation to be honest so it will come out all higgldy!
DD is 5 - year R. She has always been a confident girl, really kind with her friends (comments from preschool teachers about taking younger ones under wing etc) and likes to be friends with / play with everyone.
So - she starts school in september, the first few weeks were split classes part time, and most of her close preschool friends were on the opposite session to her, there were people she knew and played with in her session so not a problem. During these few weeks she made a new friend X.
X is kind of similar to DD in that she is quite confident and can be loud, but she can also be very manipulative of her relationships from what I've seen and seems to prefer and exclusive 'best friend' as opposed to lots of friends ... see where this is going yet?
X lives just around the corner from us, and we have basically been harangued into walking home with her most days as she would chase us to catch up etc if we had already left - this was fine to begin with, DD and her seemed good friends so its all good.
Anyhoo - this term, DD has started to complain about X more and more. We have tried just giving her tips on how to deal with it.. explaining how if you just play with one person you can start to annoy each other so maybe go and play elsewhere for a bit etc etc, and over the weeks X appears to be manipulating DD more and more ...
as an example - DD said she was going to play with a group of boys one playtime when X demanded they do something, so X does the whole 'they said they don't want to play with you'... DD very gullible, and believes it. Then we get the whole, if you don't do .... I'm telling
.... DD doesn't want to get in trouble, so does it - even if its naughty 
I have explained that as long as she is being polite and kind in how she talks to X she won't get in trouble for wanting to play with someone else but its getting nowhere.
Now DD is starting to seem less and less confident, and this week at school drop off we have had tears twice... we have NEVER had tears at school / preschool / any kind of drop off before
so I took the teacher aside (whilst DH dealt with DD) and explained that she seems to be having some troubles with X and could they maybe keep them seperate a bit today. Teacher is job share, main teacher back tomorrow - I need to broach it with her. We have parents evening next week but only a 10 min slot then, so was thinking of asking to talk to her after school tomorrow.
I don't want to come across as a raving banshee, or that I think DD is an angel / X a devil - as I know thats just not true, but how do I explain how much of an issue it is for DD to make them fully aware without coming across as a mentalist?!
sheeesh - I have just seen the lenghth of that... if anyone makes it to the end I would love a reply!