I assume that his rear sight is not adjustable. That being the case, he will need to replace his rear sight with a taller one. My first idea of replacing the front sight only was predicated on his rear sight being adjustable. So, if he replaces his rear sight with a taller one and replaces his front sight with a new one so that he will have some room to do some filing, he will be able to move his POI up to where it should be. At that point, he can calculate how much to file off the front sight to raise his POI to where he wants it.
Of course, this all assumes that his bore is centered and that the barrel is straight. If either or both of these two assumptions are incorrect, it will take a muzzleloading gunsmith to make the corrections.....if such can be made. If bent, the barrel can be straightened. If bored off center, the barrel will likely have to be replaced. It would be cheaper and better to replace it than to have a gunsmith do all the work needed to turn it.