Make sure you have an onboarding / offboarding checklist. All accounts your engineer may have access to or is an admin. Amit Kothari is doing some cool things with checklists and flows.
There might be obvious ones like AWS and GitHub, but also less obvious ones like a random SendGrid account or some build server, etc. Make sure you are also an admin for those. Unless you were also active in the development process, it might be pretty hard if he is the only one who knows all the DevOps flows or has random scripts that were never checked into version control. Make sure that stuff is documented if not already. You can use Notion to document all these things.
Make sure you have CI/CD for this project figured out, and that you can actually re-provision the prod/dev boxes responsible for this project.