jkern15674 said everything I would have said.
Hardly anyone hammers metal hooks anymore (pitons). Climbing gear has evolved way beyond that; trad gear is the ultimate in LNT climbing. Sport climbing does involve drilling a hole in the rock and inserting and glueing a bolt into the hole. They are semi-permanent. But, as said before, ususally bolts and bolt hangers are hard to see unless you are looking for them, or you're right up against the cliff where they might be. Even then, sometimes they are hard to see.
Aid climbing involves the use of hooks (fifi hook), but these are placed on features that can be hooked. They are not pounded into the rock. The climber uses them in places where there might not be any holds to allow upward progress. So the tiniest features are hooked, usually with an etrier attached (a nylon ladder). Once the climbers ascends to the point that he is no longer weighting to the hook, it is removed and probably used again higher up the cliff. If anyone is pounding something into the rock, it would be an aid climber, but this practice was much more prevalent in the 50's, 60's and 70's. The pitons that they used scarred the rock, but I have never heard of a case of erosion caused by them.
I would like to know more about your girlfriend's paper. Is it something to effect that Natives won't allow climbing on their lands, because they believe climbing will speed erosion?
Most climbers (not all) know not to climb on really soft rock, like some types of sandstone. As an unwritten rule, sandstone in general is to be avoided if it is wet. Sandstone is much more friable after it has been rained on. But climbing on wet sandstone will result in broken holds and features at worse (which is still really bad) and not full-on erosion. I strongly think that idea of climbers causing erosion is ludicrous.
"I'm not a vegetarian because I love animals; I'm a vegetarian because I hate plants." A. Whitney Brown