For a non-hosted domain, to undelete users marked for deletion but not yet purged, it is necessary to reset the users' icsStatus attributes to active. You can achieve this by directly changing the LDAP entries (using ldapmodify), or by using the Calendar Server Utility csattribute.
However, in a non-hosted domain, once the user is purged, you can only recover the LDAP server information by restoring it from a backup.