This article explains that, in accordance with the Yahoo! Privacy Policy, only the owner of a Yahoo! account can request assistance for their account.

We are unable to investigate the issues specific to another Yahoo! account on behalf of someone who is not the owner of the account. For our users' safety and in accordance with our Privacy Policy, we are unable to discuss account related issues with a third party.

If you need to address this kind of issue, we recommend that you contact the account owner using an alternate contact method, and advise them of the issue. The account owner can then follow up with us directly if the issue persists. They can contact us by sending a help inquiry from the Yahoo! Help pages.

Yahoo! account privacy and security is an important part of Yahoo! that we take very seriously. For information about our Account Verification policies and why they are important, please visit our Privacy Policy.