you can tell your friend that an original Sharps and Hawkins in "excellent" condition, with an intact worn leather barrel cover recently went for $2400 at auction.
However, "cleaning" in any way kills the value. Literally kills it. taking so much as steel wool and oil to the finish, would drop the price to a few hundred bucks.
Touch it with sandpaper or grinders and he just turned an antique into a parts gun.
yhs
shunka