There are two problems with this. First, despite the fact that the computer may be similar to the old one, it's not identical. Drivers are different, so some things may not work. Leo recommends starting in safe mode first, and remove drivers for devices that had changed, then let Windows plug and play new drivers.
The other issue is going to be Windows Activation. When he loads Windows, Genuine Advantage will check to see if his Windows has been properly installed on a new machine. Sometimes just changing a component can cause the version of Windows to be deactivated. He could call Microsoft and let them know he's changed a few things on the computer and ask for a new activation number. There's a good chance they'll say yes, but he may have to buy a new copy of Windows anyway.
Another option is for him to put the old hard drive in as a second drive, then everything would still be there and copy protection won't be an issue.