HiFi is a hobby. As with any other hobbies, It comes down to your disposable income, and how much of it you are willing to part with to get the difference you are looking for.
We all would like everyone to agree w/ us, but that seldom happens. No need to argue over it. If he refuses to keep an open mind, not much we can do.
Does your Dr friend bug stamp collectors about why they waste money on useless old stamps? Or question people with supercars when a Honda Civic gets to point A to point B just fine? Or question grown men why they play with chu-chu train sets in his garage/basement? Why drink wine when beer is cheaper? Does he think of the arts as waste of time and effort?
But let your DR friend check out a set up like this:
http://picasaweb.google.com/SpoolyT/TAA2008#5241597791669427362
I am SURE he'll hear a difference. After he hears/feesl the difference, then ask your doctor friend if there is a scientific measurement for 'sound stage', or 'texture', or 'tonality', or 'fluidty'.
Or better yet, ask him describe the difference in shades of green he sees..
Sometimes it is still hard to verbalize the difference you hear/'feel'. Science has a hard time explaining things we see/hear/feel/taste/smell...
Yes, cables are crazy expensive, because cables are crazy expensive to manufacture. VERY few cable companies own foundries to refine copper/silver and the associated manufacturing process that it takes to produce 'audiophile' grade cable.