392E question

Are you saying it is an original 392E, like ninety years old or so?  If so, repainting it would be a crime in my eyes.  If it's a repro, who cares.

Talk him out of it! Like you said, if just the cab roof gets repainted, it won't match the rest of the boiler shell.  Superficial scratches are not worth ruining an original paint job. Show him the replies that you are going to get. 

from your description it doesn't sound like it needs a repaint. they are only original once.

Keep it original.  Will certainly help keep its value. repaint 50%

Could you post some photos of it? I am inclined to leave it be, but there might be some other reason that he is doing it that might not be as obvious.

Gunmetal is tough to match especial the 80/90 year old patina.
I do a some repaints, but only on items that have been repainted or almost no paint at all with the original paint lost.Tell him to leave this one as is then to look for another that needs a complete re-build/re-paint and to bring that one back to life  and paint it as he likes. Now he will have a comparison, one could pull freight and the other passengers and could enjoy that world.
As far as a re-paint/re-build at 50% the suggested book value (states in Greenbergs?) is determined by the buyer. If it looks good and runs good that what counts. I would rather have a rebuild rather than a shelf queen with swollen wheels or zinc rot on a chassis/cab just sitting.
As El Classico states, show some photos for a better understanding.

Will post some tomorrow. Thanks for the input!

My advice would be to do a thorough cleaning, leaving the paint as-is. However, by thorough I mean take it all apart and detail it carefully. I recently did this to a 390. Took every handrail, stanchion, etc off and gave that fantastic, thick original enamel a good cleaning, polishing, then finally; waxing. It is amazing what can be done, The scratches then take on a unique character and the loco remains original. Best of luck.