Even if they were the best, most reliable employee you have ever had, change all their passwords, physically and digitally lock them out of all sensitive areas, and don’t allow them access to any of your online services.
Notify your IT providers to take them off the employee list so they can’t submit any tickets. If you hve a separate email provider, have their email password changed and any income emails re-router to a current employee.
This is not only for your protection, it is for theirs. If something happens and they still have access to your sensitive business information, they could be the first one blamed for a breach, even if they never accessed anything.
Don’t take chances – do this immediately, especially if they didn’t leave on good terms!
If you are the employee leaving, insist on these safeguards too, for your sake!