I suspect he is bragging a bit, given what he has told you. If he is, indeed casting ROUND ball, he has no business shooting deer at 200 yards, let alone 700. RB has such a poor ballistic co-efficent that velocity drops very quickly. 700 yards? Not probable.
If he is casting and shooting a conical bullet, it IS possible to get hits waaaaay out there, but that brings up other problems. First, .56 caliber is awfuly big for a conical to get any ballistic co-efficent without being so heavy that his rifle either: 1, weighs 40 pounds (possible), or 2, Recoil knocks him into the next county when he fires it. BP trajectories are very high compared to high velocity smokeless cartridges. The only common denominator of any BP propelled projectile to a .240 Weatherby is the fact that both expell a projectile from the muzzle!
High trajectorys mean you must be able to estimate range within just a few yards to hit. BP guns will knock the stuffin out of any animal that ever lived, but you gotta hit'em first. Unknown distance (700 yards known beforehand, or measured afterwards), with open sights, on a kill zone about 12 inches in diameter? Not likely, at least not repeatedly.
If you look at the shooting done by the Irish team in the first international rifle match on Long Island, New York, in 1874. they used Rigby muzzleloading rifles, probably among the finest ever made. They competed at 800, 900, and 1000 yards. Known ranges, with ranging shots fired for sight settings. Targets 12 feet by 6 feet, with a center , or bull of 3 feet by 3 feet. The best target fired, by John Rigby himself, was a 55, out of a possible of 60. Great shooting, but his bulls were still scattered over an area of 3 feet by 3 feet.
All in all, I just do not believe it is likely that he has any 700 yards kills on whitetails.