Unable to download Mavericks from my list of purchases (current OSX: 10.8.5)

Hi,

I'm currently working on OSX 10.8.5, and am having trouble downloading Mavericks from the AppStore.

I've previously installed Mavericks using my own Apple ID on another Mac some weeks ago.

When I open the App Store and go to 'Purchases', I can see Mavericks in the list of available apps. However, when I try to download, I get the message 'You have updates available for other accounts' claiming that I have to 'Sign in to (AppleID) to update applications for that account.'.

The Apple ID in question belongs to a friend of mine. I can't be sure, but he may have installed Mountain Lion on this Mac at some point. However, even when he logs in to the App Store (using Store -> Sign Out / Sign In) and tries to download Mavericks, he gets exactly the same dialog message, preventing the download from starting.

Crytpically, the dialog message still shows the same AppleID, even though that's the currently signed-in account.

I'm at a bit of a loss at the moment. I've tried varying combinations of signing in / out of the App Store (as well as quitting and re-launching it, in case that was the issue). I'm not really sure what to do at the moment. As it stands, I'm stuck with 10.8.5 with no possibility of installing the update.