@Ricky you can build a pretty slick solution for your client using CloudExtend's
Excel App. Your NetSuite saved search could provide a list of invoices and payments against them. As long as you've got a bit of Excel expertise you should be able to provide a drop down box where the end user selects from a list of entities. This would then source the open invoices and payments (filtered by date if desired) from the source data and the sheet could then be sent to the customer. You can add any number of fields to the statement that might be useful for the individual paying the invoice.
Once it's built out I'd recommend creating a macro to further automate the process, including the final step which would essentially be copying the values only since the formula references would break if the sheet was sent to the client with them on it.
We've seen this implemented for quotes pretty often but this is the first time I've seen it for invoices. Unless there is a real need to use Excel I'd recommend sticking with NetSuite's own customer statements. But if they insist on Excel this is certainly a viable option.