First, the encore has a smaller bore diameter. Secondly, your buddy may be cleaning his gun with a petroleum based solvent which really adds to gun fouling since black powder and pyrodex stick to the residue left behind by petroleum based solvents like glue. You should only use non-petroleum based cleaners for muzzleloaders like TC #13 bore cleaner followed by a swab of bore butter for rust prevention.
Anyway, what your friends is getting is what's called a crud ring. It forms between the slug and the charge. It's a VERY common problem in muzzleloading.
All you have to do is run either a patch soaked in No.13 bore cleaner if at the range, followed by a dry patch or two. If in the field, just use a spit patch followed by a dry patch.
I do this in-between every shot. You should anyway, even if your not getting a crud ring just for improved accuracy.
There are other things out there to help reduce crud rean formation. One such item is called KleanShot primers, new from Remington. They burn finer than the convential shotgun primer and are more akin to rifle primers, they are said to reduce the formation of crud rings. They are all the rage right now in competition muzzleloading.
Hope this info helps!