Is he using proper equipment or just trying to stuff the bullet in? His seating die isn't set up right. Have him loosen the seating plug, then set up the die so it just kisses the shell holder with the ram all the way up and lock it. Then adjust the seater to get the OAL he wants. If he's crimping, tell him to stop. Too much crimp can cause the case to bulge.
"...full length sizing was not done correctly..." There are multiple causes. Bad resizing is certainly one of 'em. Likely the most common too. Die is down too far and the shoulder bulges.