Submit a new build for iOS App for another account

I am an iOS developper and publish apps under my name.

I was requested by a friend to help him update his own app. He is registered as a developer himself, has valid accounts. And I would like to submit myself the new build. Of course he gave me the full source code.

He tried to add me as technical user but this was not accepted. Then I created a new mail account, which worked. What is next step ? I need his certificates, profiles,... ? Or is it just not possible ?

Of course I could update the code on my side then give him back the full directory. But not very satisfying.

Thank you.