She associates how she normally speaks with herself, and normal speech with the imaginary friend. The brain really is fascinating. I'd mention it to her doctor or anyone treating her, but not really worry unless there are other signs or symptoms of something wrong and/or she seems to be having a lot of distress. Has she always had the imaginary friend, or is that a new development?

This reminded me of a similar thing with people who stutter. When stutterers sing, they stop stuttering. Nobody really knows why, as far as I know, but I think it might be related to concentrating on something else (the music, the imaginary friend) instead of on one's own speech.