That is a Colt Model 190...5, I think it is, although it might be 1906. The grip is the style I associate mainly with the 1920's (before that they were round-topped, sometime in the 1930's Colt switched to wood). You photos (thanks for putting up clear pictures!) make it look like it's in pretty decent original condition. I would guess it is worth $300 at a minimum, but I don't know what the upper level would be. As with any sale, it depends greatly if you are selling to somebody who wants it for themselves, or is buying it to resell at a profit.
This was a classic John Browning design. It was, in fact, the gun the 25 Auto (or 25 ACP) cartridge was designed for, also by Browning. FN in Belgium made the same gun (although parts are not interchangeable with yours) as the Model 1905, and cheap copies were made in Spain by the ton. HTH!
although the author uses different model years than I do. I was surprised to see that the serial number list he includes suggests your gun was made in 1915. I also find it remarkable that according to his list, production of .25's continued steadily through World War I.
These are the Colt .25 ACP pistols commonly referred to with the name 'Colt Pocket pistol' or 'Colt Vest Pocket pistol'. They are neat little pistols too. I have seen ones in excellent condition sell for close to $500. One just went for sale as a parts gun for $120. So, considering the condition in this photo I see, I'd guess somewhere between $100-200 since it appears not to have the magazine and a replacement mag will cost around $40.
It's a Model 1908 Vest Pocket Hammerless .25 Auto. The serial number puts it as made in 1915. There were app;. 420,000+ made from 1908 through about 1948.
Neat little guns. Is your friend planning on selling it? If so, how about a picture or two of the other side......