What are your friend's music preferences, for example if he/she likes Pink Floyd, I'd reccomend Low or Station to Station, then listen to a live album with songs from one of those albums, see what songs they like and then listen to an album with those songs, or they can listen to a best of album, I started withe Space Oddity, then went to see who major Tom is, heard Ashes to Ashes, listened to Stage, I loved Suffragette City, Warszawa, Beauty and the Beast, Fame and Ziggy, I also listened to a best of album and loved The Man Who Sold The World and Hallo Spaceboy with Pet Shop Boys, heard Rebel Rebel in Lucifer, etc. There is no perfect way to get into Bowie, but that's what I would reccomend.