For what it is worth, I am sure his comment is based on many of the reasons provided here, but ultimately it comes down to a personal preference.
I would say your friend is very obviously biased towards print (all serious photographers
should be, according to some). I think his pictures are great and he composes his pictures
nicely, which shows some skill. With the kit he's got (all professional as far as I could see), the
first emphasis is on getting the picture - once that can be reliably done, then composition
comes into play (people don't buy fast lenses and ultra-zooms to compose better, they buy
them so they can get the shot in the first place). He has the same viewpoint as Monoceros,
that how he has composed should be the final result - perhaps because he is justifiably proud
of his skill.
But I think you could do him a solid by getting him to re-think this viewpoint. His water bottles,
for instance, would in my not so humble opinion look much better as a square crop. There is
filler in some of his other captures too.
As far as paper sizes go, that just means framing decisions - as alluded to by Monoceros. My
professional friend sold me a picture - when I took it to be framed, I got to make a number of
decisions about the frame - its colour & style - the backing - the matting. The standard sizes
are usually done to remove some of these decisions. And maybe go with a stock frame.
Ultimately I think it goes back to Pythagoras and number theory (where the Golden Mean comes
from). We are culturally conditioned - via the Greeks - to certain ratios. This is something else
to be considered when producing a collection. For myself, I like square crops. They have a
certain symmetry. If you wanted to 'jangle' people, you could try an off-ratio crop. Not for
most of us, but maybe something for the artistic crowd.
If you look at the Luminous Landscape guy, he considers his framing very carefully, and it is
done to strengthen the effect he is after. @Littletank, all producers are obliged to consider
their audience - in fact, this maxim is repeated endlessly to writers: know your audience.
@Rolf - too bad more software doesn't support the inside-out crop better. I do the same, but
the mechanics force me to jump through hoops to get there.