Lyric discussion by ajdedo 

Cover art for Adia lyrics by Sarah McLachlan

-- moenzzz was the closest. and Yes, it is about a woman, a close friend of hers.

The story goes -- Sarah had a best friend, her name was Crystal. Sarah stole Crystal's boyfriend, and later felt bad about it. She wrote the song Adia, and now they are all friends. END of story.

I got this info from a reliable source who knows all the three parties involved.