My guess is he is not seeing lead, but plastic build up from the powerbelt. The powerbelt uses an undersize projectile with a plastic “skirt” mounted to it. When the gun is fired the plastic expands to engage the rifling thus stabilizing the projectile. The plastic falls away soon after leaving the barrel. Powerbelts are known for ease in loading especially after a second or third shot, but the buildup caused by the plastic can be a problem. “Pellets” are merely synthetic powder pressed onto shape that easily slides down the barrel. They come in 50, 30 and 10 grain increments. They work well for fast reloads but are know to very as much as 10% in weight therefore are not as accurate as using loose powder.
Have your friend try different combinations of bullets and powder until he finds the combination that works best for him. I shoot .50 cal and use 130 grains of 777 with a 240 grain powerbelt. That combination works well for me, but may not be the best for him.
By the way, I shoot traditional and in-line. I have a custom made .45 cal Kentucky long rifle that was made for my father and passed down to me. I also have a TC Hawkin in .50 as well as a couple of in-lines. I enjoy shooting all of them.