How to allow executive assistant to sign on CEO behalf

Is this using DocuSign, or just a regular signature in Acrobat? If the latter, anyone can create a signature profile under any name, there's no validation of that. So they could just create a new profile under the CEO's name.

The only validation is of the public key against the private one.

If the CEO already has a signature profile that he wants to let someone else use it you'll need to copy it to that person's computer. You'll find it under this folder (for Windows):

C:\Users\<USERNAME>\AppData\Roaming\Adobe\Acrobat\<VERSION NUMBER>\Security\

It's a .pfx file with the same name as the profile. Copy it to the same location on the other machine, and give the user the password to use it, and they'll be able to sign using that profile.