All user permissions are managed in the Adobe Admin Console (https://adminconsole.adobe.com) and the documentation can be found here.
Projects and credentials are owned by the organization as opposed to individual users. As such all System Admins have access to all projects/credentials in the Developer Console. Developers have limited access based on the access that was granted to them. Management of Developers can be found here.
Please confirm that your colleague and you are logging into the same organization and both have System Admin privileges. If you still see a problem, please send me a DM with the name of the org, the orgID and the emails of the two system admins so I can check internally