Not sure if there is any good Arabia language module for NAV out there but I have tried what you mentioned to replicate the issue. I copied the correct text محمود الجندى as pasted as the customer name, raised an invoice and on the print i see correct محمود الجندى.
looks ok to me. Try to copy and paste the arabic text in BC from the source if you have a source in word file or online without typing it and see what happends