Override WP Class Private Function

As @jacob suggest, you can add subscriber role into that array using add_filter . add below code in active theme’s functions.php file.

add_filter( 'wf_user_permission_roles', 'wf_user_permission_roles_callback' ) ;
function wf_user_permission_roles_callback( $roles ) {
    $roles[] = 'subscriber';
    return $roles;
}