Yeah, getting the internal drive and an external enclosure is a good idea but he will need to format the drive to either FAT32 or NTFS so your system can read it. I would recommend the ntfs since this is the more modern file format on windows.
If he is no longer using the external drive that you speak of, maybe he can open it up and take out the HDD and replace it with the one you provide him.
The real issue here is the file systems and how the OS' will read/write to it.