What do you have against fresh installs? If the guy wants one for whatever reason just let him!
EDIT: Just to clarify for the OP, I wouldn't recommend a fresh install when the machine is new.
So, you ARE telling him not to do a clean install, then?
If anyone wants to do a clean install every year, every new OS, every 10 minutes: that's fine with me, and I'm not preventing them.
However, I'm merely pointing out that there are very few benefits of doing so, other than the nice clean smell of fresh bed linen.
Will there be a few Kb here and there of files that may not be necessary? Perhaps.
Is that worth a couple of hours of reinstalling the OS, then reinstalling apps that run off installers, and then cloning back your user account --
where most of the cruft is stored anyway? I'm suggesting "no".
Rewriting
all your data -- OS, apps, and user files -- on a regular basis is very likely to have a deleterious effect on a drive, of any kind.
I'm just pointing out the disadvantages: I have yet to hear of any advantages. But of course: anyone is free to do anything.
All the best