What to do when a user leaves? First, if it's an emergency, reset the password and initiate a logoff.

So technically, when a user leaves there are two places where their data will be deleted once you revoke the license or delete the user account:

• Exchange Online: Email will be removed after 30 days unless you convert the mailbox to a shared mailbox or if you make the mailbox inactive by setting either a Litigation Hold or an In-Place Hold.
• Onedrive for Business: The data will not be removed if you just revoke the license. If you delete the user account the data will be removed after 30 days.

You can use Onedrive for Business delegation so the manager of the leaving employee or an administrator gets access to the files so they can save the data to a different location.

The question is if that's legally allowed. The answer is it depends. I live in europe and it is not allowed by default. You can however, make it a policy in your company that it is by clearly defining and communicating in what way the company has access and the user can protect it's personal data.

• Remove a former employee from Office 365
• Get access to and back up a former user's data
• OneDrive retention and deletion
• EDPS: Access to eCommunications data when an employee is absent