Confirm my WoW CD key

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 Blizzard Entertainment.

(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.

There is no existing method to discover your CD key. It is not stored in your account settings on Battle.net nor is it able to be extracted from the game files.

Your friend should be able to reset his account password or recover his account via Battle.net's account recovery tools. Alternatively, call support and ask them for assistance. They'll be able to give assistance aimed at the real issue (recovering the account) as opposed to trying to retrieve the CD key.

If you call Blizzard and read them the CD key, they should be able to tell you which one is used on your account. They often use the CD key as a form of verification, so they should be able to help you figure out which is the right one to use.

It's not the best answer, but you can't see keys on battle.net, so I think it may be the only way to determine which one you used.