Source code for https://victorfont.com/formidable-forms-dynamically-hide-admin-fields-front-end/

dynamically-hide-admin-fields-front-end.php

<?php

add_filter('frm_field_type', 'hide_admin_only_fields_on_front_end', 10, 2);
function hide_admin_only_fields_on_front_end($type, $field) {
	global $wpdb;
	$frm_fields = $wpdb->prefix . 'frm_fields';
	$regexp = '"admin_only";s:13:"administrator"';
	$field_ids = $wpdb->get_results("SELECT id FROM $frm_fields where field_options REGEXP '" . $regexp . "'", ARRAY_A);
	$admin_field_ids = array();
	foreach( $field_ids AS $row ) {
		$admin_field_ids[] = $row['id'];
	}
	if(in_array($field->id, $admin_field_ids )) {

		if( (!is_admin() || defined('DOING_AJAX') ) ) {

			$type = 'hidden';
		}
	}
	return $type;
}