Celestine is my FAVORITE BE color! I use it for EVERYTHING! Highlight on eyes, on cheekbones, in the pout of lips... I love it!
As for a sub in MAC colors, I can't really think of one. Shroom is a good choice if she's looking for a not-so-shimmery color, but they're not that similar in that one is more lilac (as you said) and one is more beige to me.
I wish I could be more help. lol