20" at 25 yards or at 100 yards?? makes a huge difference in where I look. and yes if the magazine tube is not straight, then it pulls the barrel off. Even though the sights are barrel mounted, it is like having the stock bent to fit the cast off of one shooter, then having another shooter use the gun.
Just on the outside chance that all of the above checks out perhaps a close look at how the gun fits him is in order. This is the last thing to look at though. Sight adjustment first, barrel straightness second, then look at the stock and how he is holding the gun when mounted, and while mounting.