I think it is because cars have become so reliable that customers now look more at bodywork, when I started in the trade cars suffered far more with crunching gearboxes, worn clutches noisy back axles,oil leaks etc. The customer is spending what is to them all the money they can afford, so they
have to look for faults. The answer is to know when to say no, because in my experience once you start aggreeing to do scratches and dings they
will find more and more until your profit has been hammered. I have also heard salesmen say "Don't worry about the scratches we have a man who comes in and does them before the car goes out" guess what the customer expects then on handover?
Regarding the expert friend, I have always found it best to win them over rather than embarrass them. They have probably been put on the spot,
same as you probably would if one of your friends asked you to check a car, a good idea is to point out the weak points of the car (they all have them)
and demonstrate that your one is 0.k. If the expert friend is relied upon by others he could become good for refferalls, but not if you
make him look silly, tempting through it may be.