I think your friend is correct ref. 10X scope for long range shooting. I've been using the Weaver T-10 on my Model 70 300 WM 1000 yard rifle for about 30 years and never needed more power..
There is a lot of mirage between your barrel and the target 1000 yards away. Higher magnification is going to distort the target. But there is another problem you don’t see on these forms. That being the heat rising off the barrel. A typical 1000 yard match consist of one or more 20 shot matches, most with unlimited sightseers until you start shooting for record. Even the heaviest barrel is going to give you heat waves coming off the barrel. You magnify these heat waves it going to blur out the target and interfere with your ability to judge mirage and other conditions.
20-25 X is the best I've found for judging mirage. BUT: you don't focus the scope at the target; you focus it about half to 2/3s way to the target. This will give you a fuzzy target but, easy to read mirage. So you want the 20-25 power, but not on the rifle. Use a spotting scope, if set up properly, all you have to do is turn your head away from the rifle and look in the spotting scope.
Now again, here is another problem with the high power scope on the rifle that you don’t see covered in these forums. People have a tendency to focus the scope on the target, you pretty much half to if you want a clear target. If you screw up and have your scope focused short of the target, not much of a problem, however, if you focus the scope beyond the, for some unknown reason, it reverses the mirage. Meaning, if you focus the scope short of the target you can see, (for an example) the marage going from left to right, but if the scope is focused beyond the target the same mirage will appear to be going from right to left.
We played with this in sniper school and the National Guard MTU Coaches clinic to show us the reversing of mirage when focusing beyond the target.
I do use higher power scopes, a 24 X Remington, but only shooting small bore. At 50 ft to 50 yards you don’t have the problem with mirage plus 22s don’t develop the heat radiating off the barrel.
IMHO that 8-10 power is the best, never more than 12 power. Then get a spotting scope 20-25 X for judging conditions. You don’t need any more then that because you can’t see bullet holes on a target at 1000 yards with any scope. You can see the spotters on the target with an 8-10 scope.
When I was coaching National Guard Rifle teams, I use the 100 MM spotting scope behind the shooter. I never needed anything more than the 24 X eye piece that came with the scope. (It came with 16 to 32 X eye pieces).