function wpf_downgrade_role( $user_id, $action ) {

    if( isset( $_GET['downgrade_role'] ) ) {

        $user = new WP_User( $user_id );
        $user->set_role( 'subscriber' );
    }

}

add_action( 'wpf_api_success', 'wpf_downgrade_role', 10, 2 );