Just wanted to say that I remember very vividly having an imaginary friend when I was little myself! He was never 'there' (so to speak) when my friends were there, but he was frequently there when my family members were. His name was Peter, and he was a little boit older than me and we used to have lots of fun! Lol!!
As a mum now, it is strange for me to look back on. I ahd loads of social contact when I was young, lots and lots of friends and family members around me for a lot of the time, as well as nursery/school/activities, so I couldn't say that I was lonely, and the contacts I had were constants in my life. I think I just liked having company.
Please don't see it as a bad thing, and I think, be prepared for your DD's 'friend' to stay around for a while longer! x