<?php

// Facet Label = "Rating"
// Facet Name = "rating"
// Facet Data Source = "Post Type"

function fwp_edd_average_rating( $params, $class ) {
    if ( 'rating' == $params['facet_name'] ) {
        global $post;

        $post = is_object( $post ) ? $post : new stdClass();
        $post->ID = $params['post_id']; // simulate the global post ID
        $rating = edd_reviews()->average_rating( false );

        $params['facet_value'] = $rating;
        $params['facet_display_value'] = $rating;
    }

    return $params;
}
add_filter( 'facetwp_index_row', 'fwp_edd_average_rating', 10, 2 );