Cables do affect the sound, it's measurable and quite a few years ago someone here on the science forum tested and published those differences. If I remember correctly, the differences were typically in the rage of hundredths or thousandths of a dB, so clearly inaudible. With regard to the friend, there are three options:
1. Daniel is an imaginary friend, invented to try and provide credence for the typical audiophile nonsense.
2. Daniel is a real person but has an imaginary Ph.D.
3. Daniel is a real person who has a real Ph.D but is desperate enough for money that he's sold his soul to be a shill.
#1 is the most likely IMO.
While that's a valid argument, there are better arguments IMHO. The music to which audiophiles are listening has been produced in world class studios, which use relatively cheap cables, not audiophile cables. The most important cable in the whole recording and reproduction chain is the mic cable because the signal passing through a mic cable is tiny and has to be amplified many times, as much as 100 times. That signal is often further amplified during mixing and mastering and then amplified again to feed speakers/headphones. Anything the mic cable is doing to the sound is amplified far more than with any other cable and therefore should be far more obvious/noticeable than with any other cable. So what do the golden eared engineers at the multi-million dollar studios use? Mic cable which costs just a few bucks per foot!
G