zm307
You didn't say what sort of problems he is having, but more than likely they are not due to the harddrive. If that is the case, migrating his existing system to a new harddrive will not fix anything. If he is as much of a novice as you suggest, then he may have a harddrive so fragmented it is the problem. First thing I would do is defragment with something like Perfect Disk (which is what I use) or any of several other good defragmentation softwares. I asume he has run an antivirus and anti-spyware check. If not, then those are classic ways a system gets slowed down and made to perform strangely. It is possible that he has installed and uninstalled software and left a lot of stuff lingering in the registry. I have a neighbor who knows nothing about computers and in spite of my repeated warnings downloads all sorts of crap software she finds in browsing the net and then calls on me when the system gets weird to straighten it out for her. Your friend may have done the same, in which case you might want to run a good registry cleaner program like Registry First Aid. He may also just be low on free space on the hardrive; loads of uncleared temps etc. In that case you may wish to run something like TuneUp Utilities to recover space.
Even if you do end up migrating his OS to a new harddrive, you should do these things first anyway -- otherwise you will transfer over a lot of garbage -- although the migration will clear up the fragmentation.