Hi Abbi, welcome
I'm new too!
I knew that in 1977 Chris contributed to the album "Pacific Ocean Blue" by Dennis Wilson, even if she's not mentioned in it... but I'm sure your friend is right since one of the songs you mentioned can be found in the Disc Two: Bambu (The Caribou Sessions) from the 30th Anniversary Edition of that album.
I'm also sure Chris was involved in the back vocals, because Carli Munoz was already playing the piano.....I found this video:
http://www.youtube.com/watch?v=5iMiOJZyOnM
Munoz, Wilson and a chamber orchestra directed by Jimmy Haskell were at the backyard of Chris' home in Beverly Hills; Dennis sang "You are so beautiful" for Christine as a surprise birthday party present, so I think there was already a partnership between them.