A buddy of mine bought a Teslong after trying mine. He checked all his modern guns, and found some problems with insufficient cleaning. No big deal. He checked a black powder rifle, and could tell from the display that the middle third of the bore was caked thick with carbon. He's been trying to get the stuff out now for about 2 months; says he's close, but there are still some stubborn spots that refuse to come out. His normal routine was to pour hot water mixed with Ballistol down the tube, brush, and swab dry. After finding the problem, he's tried a dozen different solvents, and more scrubbing ideas than you can shake a stick at. Apparently he's going to modify his "normal" routine now.
I went through something similar to your friend's plight. What finally worked was to plug the barrel at one end and pour eze-off oven cleaner into the other and let it stand upright for 24 hours. Then carefully tip it up and drain the goo. Then take a bronze brush and get what you can out of the remainder. Two treatments were enough for my barrel. YMMV.
Afterwards you need to clean thoroughly with alcohol and then whatever gun oil you usually use. I hope this helps.