<?php
/** basic string replace to add nofollow to pager html **/
add_filter( 'facetwp_pager_html', function( $output, $params ) {
$output = str_replace( 'a class="facetwp-page', 'a rel="nofollow" class="facetwp-page', $output );
return $output;
}, 10, 2 );