As I have recently found out, having a team member leave a shared Google Drive can be infuriatingly complex. The best way to handle a shared Google Drive is by using an admin account for the organisation.
All of the folders within a share, should be managed by this one account. If they are created by the key member representing the organisation, then the sharing permissions for the folder can be managed by the admin. The admin can remove people from the share, or share the folder or files with other people.
Other team members should not create new folders within the Parent folder.
Other team members should, as a matter of course (for their sake as well as the organisation, upload folders, but as a matter of habit, make the owner of the file or folder the orgnisation's admin account.
If you already have an active share with hundreds of valuable files, between a number of people, then you can prepare for future issues (someone moving on) in a couple of ways. There may be better ways, please do share them in the comments if you can think of practical ones.
DISCLAIMER The following advice is provided with no guarantee of success and assumes the person responsible understands fully the process and takes all necessary precautions to ensure data is not lost.
DISCLAIMER 2 - Google forms associated with Google Spreadsheets are likely to become disconnected using the Gung-Ho method.
DISCLAIMER 3 - Think about everything first.
The Gung-Ho method.
Ensure Backup and Sync is running and make sure it's fully synchronised with all of the folders and files associated with your google drive.
Move the shared folder to a backup device.
Login to your Drive account from a browser, Delete your the shared drive folder and ensure backup and sync runs fully as well.
After sync has completed. Ensure your Bin on drive is empty, and your 'shared with me' contains none of the original content.
The final step is for the admin account to re-upload the folder, all contents will now show the owner as the admin account.
The Surgical method
Ensure every team member is aware of the problem.
Each team member should log into the share and change every folder, every file by transferring the ownership to the organisations admin account.
This is much more labour intensive, and the admin account will likely get hundreds of emails advising them that they're the new owner of content within the share.
If a team member leaves and is the owner of files and folders within a shared parent folder. When they leave (the share) or delete the folder from their drive, some of those files and folders will be deleted, and some move from their drive to their shared with me, and others will move to their 'storage'.
The work that they contributed to the project(s) will be gone.
So it either needs to be uploaded again (the Gung-Ho method) or have a new owner (the surgical method).