I have explained the rules for 1040 vs 1040NR.  You may like to do a bit more homework to understand the application or refer the work to a specialist.

As for authorization to work, if your client was authorized, he/she would have been issued an SSN and would not be using an ITIN.  Enough said.  I am not here to convince you but thought you should be aware for your own practice protection and to advise your client of the potential issues.