Change first name last name to last name, first name in index

<?php
/** change names formatted "John Smith" 
 ** to display as "Smith, John"
 ** in facet's index
 **/

add_filter( 'facetwp_index_row', function( $params, $class ) {
    if ( 'personfacet' == $params['facet_name'] ) { // change 'personfacet' to name of your facet
        $name = $params['facet_display_value'];
        $name = explode( ' ', $name );
        $display_name = array_pop( $name ) . ', ' . implode( ' ', $name );
        $params['facet_display_value'] = $display_name;
    }
    return $params;
}, 10, 2 );