Once an expense claim has been submitted, the approvers associated with it cannot be changed. Therefore, there are a few options as below:
Set a user to approve on behalf of the leaver. it may also be necessary to change the approval route to add/remove members and details of how to do that can be found in the link in point 2.
Ask the user to unsubmit the claim, amend the approval route to add and remove the pertinent members, and then ask them to submit it again.
Request that an administrator temporarily re-activates the leaver's Evo account to allow access via a password change. Once the claims have been approved, then the account can be deactivated if required.
This is not really advisable unless the email address used is for the company domain so that the password change can be handled internally with the help of the IT department.
Once the claim can be viewed for approval, it can be processed in the usual way.