"his laptop is dying and very slow"
that might mean his hard drive is bad, or that he has too many programs/malware installed, in which case you would not want to put his drive in your laptop.
But to answer your question, it depends on how different the chipset drivers
are between the two machines. But if you have the Windows 7 x64 install dvd, you should be able to do a repair install if windows is not able to boot after switching hard drives.