In Exchange System Manager navigate to your public folder store and right click and select properties. Under the security tab add the user name with full access. You can then move the shared calendar back into the public folder from the users mailbox. When finished, remove the user name from the security properties.
Only admins can move something from the public folder store so you may want to consider removing this user from any admin groups. Non-admins can only link/copy public folder items and not move them.