Not a vast amount any more. I've seen even error coins, theoretically the most desired brass coins but not the rarest, only make 1.1 BTC or little more on here.
https://bitcointalk.org/index.php?topic=5216779.0Search this section for more. I haven't seen any 1 BTC brass coin go for more than 1.2 in recent times.
If your friend goes on Ebay they may achieve much more than coins go for on here. Whether those published prices are successful or don't end in disaster is another matter. They may point to it and demand more from you but good luck if they choose that route.