Well I see Big5 has beat me to this. I have been shooting a .340 Weatherby for 20-25 years and I have never found it to be a problem. I have also shot a .416 Rigby a good deal.
My .340 is a Sako action with a Heym barrel and a McMillan synthetic stock. It is not a heavy rifle and yet I do not find it to be a problem, even from the bench and I generally shoot the heavier 225 and 250 grain bullets out of it. Now mind you my wife says I tend to be numb and insensitive, so that could be an issue.:)
The .416 Rigby I used a fair bit was a CZ and it most certainly was comfortable to shoot.
Like Big5, I also suspect the issue with your friends .340 is stock fit and that may be compounded by how he handles the rifle.

