Index lat/long from an ACF relationship -> map field

<?php

/**
 * 1. Add to your (child) theme's functions.php
 * 2. Assumes a facet named "doctor_location" w/ an ACF relationship field data source
 * 3. Assumes that the ACF Map field name is "location"
 */
add_filter( 'facetwp_index_row', function( $params, $class ) {
    if ( 'doctor_location' == $params['facet_name'] ) {
        $location_id = (int) $params['facet_value'];
        $location = get_field( 'location', $location_id );
        $params['facet_value'] = empty( $location ) ? '' : $location['lat'];
        $params['facet_display_value'] = empty( $location ) ? '' : $location['lng'];
    }
    return $params;
}, 10, 2 );