I'm not an expert on this or much of anything else, but I'm interested in these pistols and hope to eventually acquire one. The fellow you want to talk to is forum member
@Smokey Plainsman . He had one of these and wrote it up here on the forum a while back. That's his story, though, and we will let him tell it.
There was a great, short article on the
U.S. Model 1842 Percussion Pistol in the November 2020 issue of
The American Rifleman, in the "I Have This Old Gun" column. Click the link and it will take you to the article online. The one they reviewed had damaged markings and a battered nipple but was otherwise functional and mechanically sound. They said it would be worth $550 to $650. In similar condition but with legible markings, it would bring between $850 and $950. Based on the information I have, your friend is offering you a pretty good deal. I do see a crack in the stock in the panel opposite the lock, but this should be repairable and the gun would likely be okay to shoot, anyway. Markings are damaged but generally legible. Otherwise, the gun appears to be in very good shape.
As for shooting that beast, Mike Cumpston did a field/range test with an original and wrote it up here:
Web Blast: The Aston/Johnson Martial Pistol of 1842. Mr. Cumpston is an accomplished pistol shooter and "gun writer," and he did a good job with this report. One thing to consider, as Mr. Cumpston pointed out, is that this is a pistol intended for use on horseback. You don't want a hair trigger on a pistol if you're perched on the hurricane deck of a dancing war pony. Mr. Cumpston's pistol had about a 15 pound trigger pull, which is evidently normal for these guns.
The photo above, courtesy of the NRA, shows how the pistol is put together and reinforced. There are a lot of them still out there, probably because they were built to take some abuse. The butt is reinforced front and back so the weapon could be reversed and used as a club after its single shot has been fired.
So, it looks like your friend is offering you a good deal on an interesting, fun, and historically significant old gun. If you have the money, I don't think you can go wrong. If you decide to pass on it, I would be interested.
Best regards,
Notchy Bob