Talking to him - educating him, if you will - after the sale is probably worse than useless. He bought the gun, which means that in his mind he is sold on the gun. Your education would be fighting his confirmation bias, i.e., the conviction he's bought the right gun, which came after deciding this was the right gun for him. Not only would you be spitting into the wind, but you'd likely be insulting him (at least in his eyes).
Talking to him before the sale might have a more welcoming reception, but only if he wanted the help. Giving unsolicited help is very, very rarely a good thing to undertake. How one would approach the kid would be very important to the ultimate result.
The salesman feeding him a line of crap about the pitting is either at fault for defrauding his customer, if it was a deliberate deception, or too ignorant to be giving expert advice if not. In either instance, I suppose you would be within your rights to tell the store management about what you saw, assuming your relationship with the owner was sufficiently developed that he would listen to you. It is entirely possible that the store owner/manager was ignorant of salt-wood Brownings, or overlooked it inadvertently, too. It should be unlikely, but it could happen. Or they could have overlooked checking the serial number. Or they could have decided it wasn't worth quibbling over and any disclosure they intended was either forgotten or not communicated well.
Or the kid could have known exactly what he wanted and found it there, known about the salt wood on his own inspection, and took it on anyway.
