I don't know if anyone will be able to help much on this but I thought I would give it a try.
A friend's 6yo dd is very difficult. She will rarely listen when being told anything and is very disobedient. This is not all the time - when she is nice she is lovely and I guess she could just have a very independent and fierce personality. However I think there is some other problem. When told to do something she will often start it but then sit down and talk to her dolls as if she is in a world of her own. She will also play up generally much more than any other 6 year old I've met. To be quite honest if I didn't know the mother I'd say she had some sort of problems at home (lack of attention or discipline or something), but as far as I can see the mother is doing everything right!
The symptoms are almost like some attention deficit disorder (though she's not really hyperactive). Could this type of behaviour be caused by something like a food intolerance?