You obviously have not tried Sabayon which installs about 15 GBs before you
add anything to it. It is one large distribution!

In Kubuntu, I have only installed 11 Gbs on my very full root partition. I
have just about every desktop environment installed and lots of
applications.

My advice is that with 14 GBs I would keep it lean. If he is doing lots of
installing the cache fills up quickly and multimedia can chew up lots of
space with temp files. I would take advantage of the Computer Janitor and
Kleansweep. I also like Filelight which shows you where space is being used
the most.