I thought it looked like the (broken-off) brass stem-&-ball of a civil war Enfield cartridge-box's flap-closure finial when I saw the photo. Then reading down through the followup posts I see that Richeysd76 has kindly posted a photo of an intact Enfield cartridge-box finial. So, that is what I think your friend's find is most likely to be. Being found at a civil war site is what makes me lean toward that ID, because there are other similar looking ball-on-a-stem objects. I'm not 100% certain, that's just my "best guess," based on its shape, its size in comparison with the dime in the photo, and the dig-location.