Index latitude / longitude of related post

<?php

/*
 * 1. Add to your (child) theme's functions.php
 * 2. Replace "location" with the actual facet name
 * 3. Replace "latitude_longitude" with the actual custom field name
 * 3. Re-index afterwards
 */

add_filter( 'facetwp_index_row', function( $params, $class ) {
	if ( 'location' == $params['facet_name'] ) {
		if ( ! empty( $params['facet_value'] ) ) {
			$related_post_id = $params['facet_value']; 
            $coords = get_post_meta( $related_post_id, 'latitude_longitude', true );
            $lat_lng = explode( ',', $coords );

			$params['facet_value'] = $lat_lng[0];
			$params['facet_display_value'] = $lat_lng[1];
		}
	}
	return $params;
}, 10, 2 );