This is already done by Odoo itself, with the only condition that the sale orders need to be in manual state(when you confirm the sale order as a sale and get the state to be invoiced). in the tree view you could select all the sale orders that you need to invoice and in the more drop down there is an action that reads "Make Invoices". Clic on it and you will see a wizard to create the invoices for the selected sale orders with the option to group the invoices by the client, meaning that you don't need to worry about if the sale orders are for the same customer or not, with grouping invoices you will end up with one invoice per client.

Hope this helps