He'll have to install the OS (Windows I'd guess), then probaby install whatever applications he use and maybe move his data over in some way (if he doesn't already store it on a separate drive - using a secondary drive or an external enclosure for the old drive).
I cloned my old hard drive to a new partition but it was a pain in the ass I don't want to go through ever again.
And it's often a better choice to start anew rather than bringing any (possible) software-related problems to the new disk (unless you're certain that the old installation is in a good shape), then move over documents and other files manually.
but he doesn't use a mail client anyway
I suspected as much, this is often the case, just making sure you're aware.
Personally I prefer a real mail client (with IMAP connection and server storage, thankyouverymuch, POP can go dig itself a hole, as can most web-based mail clients I've used), and I have a few friends who have had the same POP mail account since long before webmail became ubiquitous and who still prefers using their ISP's space-limited mail service for whatever reason (difficult teaching old dogs new tricks, I assume), even if it means only having part of their mail on each device.