I see I am going to be the unpopular one on this thread, but here goes:
This : My DD is very happy at school, doing well, popular with big friendship group, good social life out or school etc but friends DD is very unhappy. My friend says her DD is picked on at school although my DD says people find her very bossy, rude and difficult to get along with and she doesn't really do anything to help herself. She has no interest in music, hair, clothes, sport, boys and DD says she finds it hard to talk to her now as they have very little in common. ... made me feel uncomfortable.
The friend says her DD is picked on at school. For "picked on," read "bullied."
OP's DD says "people find her very bossy, rude and difficult to get along with and she doesn't really do anything to help herself. She has no interest in music, hair, clothes, sport, boys"
Sounds to me like she is being bullied because she doesn't fit in and she, the victim is being blamed - "doesn't really do anything to help herself."
To me it's a classic exchange:
Person A: I am being ignored and picked on.
Person B: Well it's your own fault as you don't do anything to help yourself. You are not interested in shallow 12 year old pursuits, what do you expect?
Person C; Someone needs to show person A how to be more likeable.
How does the girl get to be bossy if she has no friends? Who does she boss about?
You have known the girl since she was tiny, OP. What do YOU think of her personality?