Now that we are well into our second page of discussions and speculation about a rifle that we do not know the maker or how the breech is chambered, I'll add a bit of my speculation. A better answer could be given if we knew who made the friend's 0.54 caliber percussion rifle or what the design of the breech was. It would also benefit to know a bit more about the other rifles that do not smoke after firing.
My speculation is that he has a chambered breech. I also speculate that he cleans his rifle with a lot of liquid cleaner and stores his rifle after cleaning with a lot of oil left in the chambered breech that can't be dried out. Then the rifle is stored muzzle up so that whatever oil is left in the breech will leave large amounts of gelled oil residue. When fired, some of this gunk will continue to burn and smoke will be emitted from the muzzle for a short time after firing. I recommend for the friend to store his well cleaned rifle muzzle down so that the oil can drain from the breech and not build up that residue of smoldering gunk that burns after a shot is fired.