Moving L4D2 onto another mac

Been looking around the forum tonight for an answer and couldn't really find anything. Wondering how I would go about moving l4d2 to another mac. I have it installed on my mac, and i'm gonna try to get my girlfriend into the game (since its so cheap right now figured it was worth a try). So i'd like to transfer the files over to her mac instead of her having to download the 11 GB, and i was wondering what i'd have to move, and to where. And should i transfer first, and then buy the game on her account, or buy first then copy.

Thanks in advance