When viewing registrations linked to a specific event a checkmark is shown on the primary registrant is the transaction is complete, this function extends that to show it on all registrants within that transaction.

checkmark_on_all_column_TXN_paid.php

<?php //Please do not include the opening PHP tag if you alreayd have one.

//This function adds the checkmark to all registrants linked to a Complete transaction. By default EE only shows a checkmark on the primary registrant.
add_filter( 'FHEE__EE_Admin_List_Table__single_row_columns__column_TXN_paid__column_content', 'checkmark_on_all_column_TXN_paid', 10,  3);
function checkmark_on_all_column_TXN_paid($content, $item, $object)
{
    // If primary registrant, return whatever EE already has set.
    if ($item->count() === 1) {
        return $content;
    }
    // If not the primary registrant, pull the transaction object.
    $transaction = $item->transaction() ? $item->transaction() : EE_Transaction::new_instance();
    // Check if the amount paid is >= total, if so return a checkmark.
    if ($transaction->paid() >= $transaction->total()) {
        return '<span class="reg-pad-rght"><div class="dashicons dashicons-yes green-icon"></div></span>';
    }
    // Return an empty space if all else fails.
    return '&nbsp;';
}