J.G.
I think some emagination is a good thing, but sometimes it can become destracting. It is a good idea to talk to her counselor so that she is aware of what is going on & see if any issues are causing it.
I remember my baby brother having an imaginary friend & my mom had a spot at the table for him to sit at dinner w/ us. She would even put a plate & cup there for his friend. But it was something we worked through w/ him... his friend appeared after we lost our brother (which was 2 yrs older then my baby brother). Both the boys were together when the accident happened & we lost our brother - so my baby brother saw it happen. His friend helped him deal with what happened... so for us it might be a lot different then your daughter's situation.
Follow your heart - as a mom, you do know best for your child. If you do that - everything will be fine.