Is there a way that someone else can be authorized to enter time for someone who is no longer here?

If the person has not been separated, their TS Approval Manager can log in for "Subordinate Resources" - option is available in PR only for TA Approval Mgrs. If the person was separated, they can only adjust the cost via an expense.

Assign and activate a delegate

Whether you are a resource manager or project manager, you can assign a delegate user to process your timesheets when you are not able to process them yourself. After selecting a delegate for your resources or projects, you can activate and disable the delegate's approval rights at any time.