Go to your friend's house and deauthorize your friend's computer for your account. It is apparently set to automatically download new purchases and it has your AppleID. You will have to deauthorize it in person or deauthorize all computers authorized to your AppleID.
iTunes Store: How to enable Automatic Downloads - http://support.apple.com/kb/HT4539 - "iTunes 10.3 3 and iOS 4.3.3 or later (iOS 5 or later for iPhone (CDMA Model)) enable you to automatically download purchases from the App Store, iBookstore, or iTunes Store instantly to your other computers and devices."
Incidentally, if your friend has an AppleID your friend will now find the computer can't be authorized to his/her AppleID for 90 days.