Something almost has to be totally screwed up with his reload....whether powder, bullets or primers. Is he an experienced reloader? Need to root out the problem, but doubt it is a "shot out" barrel.
For a powder check, try another powder and chronograph and it. Try another reload or two with the same load.
Verify the scale accuracy by weighing some bullets.
Try to insert a new bullet in the muzzle....should be a very tight fit and he can't push it in to verify .264" bullet and muzzle diameter.
AS per primers, try a different lot or brand of magnum primers.
As per the barrel....push a new .264" brass bristle brush down the barrel and feel for loose spots or spots that snag the brush (carbon?). Should be a snug fit and push fairly hard with a brand new brush with bore cleaner on it. Barrel likely needs cleaning anyway.
Scales should be verified with check weights. There is too much actual weight variance from bullet to bullet to use them to verify the scale.