Moomin

<?php

function my_searchwp_extra_metadata( $extra_meta, $post_being_indexed ) {
	// index the author country
    $author_country = get_field('my_countries', $post_being_indexed->post_author );

    // it's stored as an array so let's get the actual text
    $author_country = is_array( $author_country ) && count( $author_country ) ? implode( ' ', $author_country ) : '';
    if ( ! empty ( $author_country ) ) {
    	$extra_meta['my_author_meta_country'] = $author_country;
    }

    return $extra_meta;
}

add_filter( 'searchwp_extra_metadata', 'my_searchwp_extra_metadata', 10, 2 );

function my_searchwp_author_meta_keys( $keys )
{
    // the keys we used to store author meta (see https://gist.github.com/jchristopher/8558947 for more info)
    $my_custom_author_meta_keys = array( 
        'my_author_meta_country' 
    );
    
    // merge my custom meta keys with the existing keys
    $keys = array_merge( $keys, $my_custom_author_meta_keys );
    
    // make sure there aren't any duplicates
    $keys = array_unique( $keys );
    
    return $keys;
}
 
add_filter( 'searchwp_custom_field_keys', 'my_searchwp_author_meta_keys', 10, 1 );