Need exact diameter and weight to confirm authenticity either way. Comparative ping tone test should help.
Micrometer screw gauge and accurate electronic balance are the tools for this. Every coin dealer should have these basic coin measuring instruments. If he had any doubt, he should have tested it himself. If he hasn't got those instruments, tell him to buy them. He should have more expertise than just about all of his customers.
Picture of the coin looks 'pasty', but that alone should not condemn it.