When shooting, you're checking that the scope is level to the ground, not whether the reticle is level to the scope. The Sine of 1 degree for a 1 meter long hypotenuse is about 1.75 cm, so it's going to be measurable/noticeable using a plumline. 5 degrees will be about 8.71 cm.
What you as a shooter can hold "under field conditions" is irrelevant, especially when you start talking about holds for distance. The 83" hold for a 6.5 CM at 600 yds would be off 2.9" of windage for a 2 degree error, or about as much as blowing your windcall by a few mph. If his is visibly canted to the point where he doesn't need to use trigonometry to determine whether it's canted or not he should send in the scope.