There is only one administrative or Team leader per developer account. So you really need to plan on the policy for sharing use of that account from the beginning, if the required activities of the agent need to be split up among multiple parties, if you can't have one party capable of doing everything.
A shared account can be created from the beginning (either by the owner or the developer). I recommend an ADC account be created just for this purpose, instead of just using the owner's personal account and email address ( e.g. instead of mary.smith@sample.com, create and use iosdeveloper@sample.com for enrolling as an iOS developer. )
Account credentials can be "loaned" (perhaps with password changes after use).
You can be given remote access (VNC/RDP) into the owners PC or Mac (or more secure yet, a VM session) as or after they log in.
You can talk the owner though the process over the phone (or video chat, etc.).
Or, the owner can learn how to get certificates, and build or resign and submit apps themselves, perhaps using a comprehensive script.