I will not answer to you that your advise is "one of the most stupid thing i have ever heard".. i'm not used to be spoken like that in my life and i don't speak to people like that neither
You may consider i'm a fool but with scott pine, with my bonsai friend, we often cut almost all the rootball except the pivot (the big vertical root Inside the soil) when we prepare tree for next spring. we let 1 big root like the pivot or another big root deep in the soil, we cut every surface root and we put back the original soil and water with organic liquid solution + fertilize with organic solid fertilizer. Sometimes the trees are very weak, with few and small needles, and we come back next spriing to see the rootball : the tree are alive and there are new white roots around the rootball. If the tree stays in the same place another year, with enough sun & fertilizer, we can see that new shoots are bigger next spring.. i don't say the owner should cut all the roots of this fir, but i think that cutting some roots wouldn't change anything. Maybe i'm wrong, i disagree because i'm passionate, but i try to stay correct in my formulations