It is possible to clone an existing invoice of inactive customer and save the invoice, practically bypassing the inactivation.
Find the invoice in
Sales Invoices tab and then you can clone it.
Making a customer, supplier, bank account, cash account, or inventory inactive does not remove it or transactions referencing it from the system. It just removes it from dropdown lists when entering transactions and to the bottom of tab lists. The purpose is to reduce clutter and scrolling, not prevent other functions.