Help me prove him wrong. "I want the instrument itself to speak the emotion of sadness," he says. Link me some good songs that show just how sad ukuleles can be.
AARRRRGGGGGHHHH. Not 'this' again!
For nearly 100 years, the media has propagandized to death that 'the ukulele is a HAPPY instrument', this is not only wrong, but also stupid and evident of lazy thinking, if any thinking at all.
The instrument has NO 'feelings' period. It is a piece of dead wood that comes alive with the PLAYER. It is the PLAYER that has feelings and if good enough of a performer can evoke FEELINGS in the LISTENER.
I hate to be that guy...but it seems like your friend is woefully misinformed about how MUSIC works and might be ignorant of basic songwriting constructs and how the song, the instrument and the musical delivery of the performer all interact to create the mood or feel of a song when it is performed. Either that or he is trolling you and trying to make you get all fussed up while looking for evidence to refute his claims.
Some people like this are broken in their mind and cannot be 'fixed' or convinced otherwise. Not all people are this way of course, but some folks are simply unable to understand because they lack the capacity to understand...
This is different from folks who are also either willfully ignorant and/or maliciously ignorant, but that's a whole other topic.
Just go on YouTube and put in the search box 'sad song ukulele' and then email him the link to that page. However, this may even be futile unless your friend has an open mind and does not hold on to confirmation bias.
Good luck!
(sorry for the rant, it was not directed at anyone in particular, I just have a low tolerance for repeating the ignorance and propaganda of fools, one such example is that 'the ukulele is ONLY a HAPPY instrument', which is COMPLETELY different from 'the ukulele makes ME happy')