Stevie's song "Touched By An Angel" is most certainly about Christine. Christine is the "someone standing at the side of the stage, not standing in the shadows I see her face. Glowing in the darkness in her in her own angel ways"
. That's really one of my very favorite Stevie songs... and one of the neatest songs that any of the band members have written about the band itself.
On the other hand, I just can't point to any one single song of Christine's and say it's about Fleetwood Mac or Stevie.
I've never heard of "Friend" being about Stevie. It's an intriguing thought... especially with all the moon, planets, and mysticism references. I guess I always figured that song was about a failed relationship.