Can the Mavericks install binary be shared between two different apple accounts?
Solution 1:
Yes, this should be fine, although I have not tested it specifically. After you install another copy of Mavericks for your friend's computer, the first time your friend logs into the Mac App Store/Software Update on his own Mac with his Apple user ID and password, it should register that he has a functioning copy of Mac OS Mavericks.
To make sure, it might be better to first have your friend log in to the Mac App Store to "purchase" the free download for the Mavericks installer, and then cancel the download as it begins. Then you can copy your installer to his hard drive (onto his existing installation of Lion or Mountain Lion) and run it from there.
I used a similar method with Mac OS X 10.7 Lion and Mac OS X 10.8 Mountain Lion; in that case, I had friends pay for the upgrade in the Mac App Store, without actually downloading anything. Then I transferred my copy of the Mac OS X installer to their Mac from my external hard drive, and I ran an installation in the usual manner. Apple and the Mac App Store did not care where the installer had come from, as long as the registered user had paid for the download beforehand.
But in this case the Mavericks download is free, so even that is no longer a problem.
Solution 2:
Yes, you can reuse the downloaded installer across Macs, even if they're using a different Apple ID account for The App Store. The installer contains nothing in it that prevents it from being used this way.
Data about App Store accounts comes from existing user data on the machine during the upgrade, it's not embedded in the downloaded installer.
The legalities of doing this seem fine: the upgrade is free and the user is required to accept the EULA when they run the installer application, before the install begins, so you're not bypassing some acceptance of this agreement by sharing the installer instead of downloading it from the App Store.
Edit and field test: A friend used my pre-downloaded copy of the installer on his MacBook Pro tonight and it worked just fine.