Use the user ID and WP_User:
$user = new WP_User( $user_id );
print wp_sprintf_l( '%l', $user->roles );
Update
/**
* Get user roles by user ID.
*
* @param int $id
* @return array|WP_User
*/
function wpse_58916_user_roles_by_id( $id )
{
$user = new WP_User( $id );
if ( ! is_array( $user->roles ) )
{
return $user;
}
$wp_roles = new WP_Roles;
$names = $wp_roles->get_names();
$out = array ();
foreach ( $user->roles as $role )
{
if ( isset ( $names[ $role ] ) )
{
$out[ $role ] = $names[ $role ];
}
}
return $out;
}
Usage example:
print '<pre>'
. htmlspecialchars(
print_r( wpse_58916_user_roles_by_id(1), TRUE )
)
. '</pre>';
Array
(
[administrator] => Administrator
)