If you preview a statement for a client from the Statements screen, it lists all the open invoices for that client.
You can also do this using the Statement report. To do so, go to the Reports screen and select Statement under Billing. You can customize the report with various filters and options. For example, you can filter the report for a specific client and transaction period, and choose to show only open invoices.
Yet another way of doing it is to filter the Invoices screen. Select the filter option on the screen and add the filters Client (setting it to the specific client) and Open (setting it to Yes). Check CORE Help Center for details.