Once a key is used, it is permanently associated with an account, but you won't be able to look it up anywhere:
Should I keep my authentication key?
Yes, the authentication key can be
used to verify an account if other
information (such as the Secret
Question Answer) has been forgotten.
However, you do not need the
authentication key to reinstall the
game. After you have established your
account, the key is permanently
associated with your account but is
not available for disclosure by
(Source: Authentication Key FAQ.)
However, if he uses the wrong key, it'll simply tell him that the key has already been claimed. There are no negative consequences to using a wrong key, so give him both and one of them should work fine.
Edit: As @wikwocket pointed out, a potential downside is that your friend could become privy to your own key. If that's a concern, one way to guard against that could be to try a recovery process on your own account and input an account key -- if it goes through, it's yours and your friend has the other one. Or the other way around.