There are two ways to access a third party's AdWords production account:
1) If the third party has a developer token and are willing to share the token and OAuth credentials, use their token and OAuth credentials to access their account.
2) Alternatively, you can
apply for basic access on your own developer token and then ask the third party to
grant you access on their AdWords account, using a new email id (not associated with an existing AdWords account). You can then
create OAuth credentials yourselves, and access the account using you own developer token and the newly created OAuth credentials.
You can choose either option for test accounts too, only that for option 2, your existing developer token already has access to test accounts (i.e. you don't need to apply for basic access).
Hope this helps. Please feel free to revert in case you have more questions.
Regards,
Vishal Vinayak, AdWords API Team