You are correct, manual sharing is available for the record owner, any role in the hierarchy above the record owner, and the system admin.
From Salesforce , You might not see the button if you’re not logged in as the candidate record owner
or system administrator. The button is available when your organization-wide default for candidates is set to Private or Public Read Only.
So you might have confirmed that the sharing
button is available when you logged in as admin. Try going to Manage Users|Users
and login as the requesting user, or ask him to confirm that he can see the sharing button.
If the sharing button is not available, make sure that Manual User Record Sharing
in Sharing Settings
is checked.
Sharing
button exists in Page Layout