He doesn't need HDD and Case, he could use old Case and HDD and save money. He also, doesn't need to buy a new Windows cause he has a Windows 7 key. He could try download Windows 10 from the Microsoft website and use Windows 7 key to activate. It used to work like that a couple month ago for sure. Although, I haven't tried it after free Windows 10 upgrade got stopped.
So, considering all that then his build is roughly $500
If his old PSU is around 550W then he could drop it off too and instead get a better GPU and CPU
Like this:
http://pcpartpicker.com/list/ZB69r7