I'd love to know for sure as well, but as someone who ordered a MBA from the UK education store today, it seemed, from what I was told, that it is linked to the serial number. I'd check with Apple though. If you're buying from a friend, then as long as they don't mind having it sent back in their name, it won't be a problem. Also remember that it is just an extended warranty, not the full Apple care, but I suspect that the tech support is better on this forum anyhow.
David