The reports can be customised as required. Potentially you could create your own copy, or modify the existing report at /modules/reports/client_statement.php report.
A customisation could add a new variable for the invoice number and adjust the output to use that instead of the $invoiceid. For example:

$invoicenumber = $result->invoicenum;

Related documentation: https://docs.whmcs.com/Reports#Customising_Reports