If you're dropping dove and having troubles with ducks...you're shooting in front of the ducks. You don't have to lead them as much they aren't near as fast as a dove, cut your lead in half for what you think you need would be my guess.
It's more likely than not that he's missing shots from too little lead than too much.
The shot string from a typical 3" 12 gauge shotgun shell containing #6 steel shot moving 1400fps, is '6 to 8' inches in length as it exits the barrel, growing in length the farther it travels through the air as it moves towards the target, at a velocity averaging 1100fps and 5'-8' feet long in length at 20 yards, slowing to 750fps and 12'-16' in length at 40 yards. Steel shot doesn't deform like lead, resulting in a shot string with a smaller diameter than that of lead shot, A steel shotstring is roughly teardrop in shape, point facing forward, like an inverted cone, with the bulk of the shot at the rear of the shot string. Lead shot is easily deformed and mishapen individual lead pellets are prone to take wildly divergent paths, the reason lead patterns wider than that of steel when shot using a given size of choke. Both the length and width of the shot string significantly increase the chance of hitting a bird. One reason we don't hunt hunt waterfowl with rifles.
When fired, the shot in a shotgun shell is significantlly larger in volume than a bullet, increasing the odds of hitting the bird. The birds path can intersect that long cloud of shot at any point along it's path, the front of the cloud if the shooters lead is short, or the rear of the cloud if the lead is long. bird expands longitudinally and widens as it moves toward the target, in a long, thin, teardrop/cigar shaped pattern, growing wider and longer the farther it travels until gravity and drag take affect.
I've seen high speed video of ballistics tests on various gauge shotshells fired at static gell targets, clays and live, flying birds. The videos were enlightening to put it mildly. It put to rest any doubts I had of the importance of patterning the shotgun, chokes, and shotshells you intend to use in the field to baseline their performance and determine where POA needs to be to acheive a specific, consistent POI.
A specific brand and spec shotshell that performs well with a specific shotgun brand/model/choke , may not shoot worth a damn in a different shotgun. It doesn't take much time and effort to find a combination of shell/choke that works well in your shotgun. When you get it dialed in, hitting what you're shooting at is almost effortless.
Winchester Ballistician E.D. Lowry ("The Effect of a Shotstring," American Rifleman, November 1979)
Gerald Burrard (The Modern Shotgun, Chapters V and VI)
http://www.gunnersden.com/index.htm.shotgun-shotstring.htmlBob Brister (“Shotgunning, The Art and Science”, 1976)
http://randywakeman.com/lowryshotstring.pdf