You are a just man with sound reasoning.
I can't tell from your post if this is a hypothetical or you are weighing in your mind whether to do this? Assuming this is hypothetical I can see two parts of the question; 1) is just people getting bad service from an incompetent or lazy pro, and 2) is perceived dog abuse. Unless the abuse was so over the top and was observed first hand over many sessions, I might anser truthfully what I saw if asked directly and or offer my opinion if my friend was considering this pro, but otherwise keep my mouth shut.
I have witnessed training sessions where through no fault of the trainer, the wheels fell off the wagon and the session rapidly went downhill, to the point where there was way more pressure involved than this trainer ever uses, but there was no other way out that wouldn't have more lasting ill effects. If somebody happened by right at that time they would likely think the worst about this trainer, when in reality he is on the softer side of the spectrum, can read dogs like a wizzard and is more than fair in giving dogs the benefit of the doubt. In this case it was a very hard headed dog that the trainer knew could handle pressure most dogs couldn't and like I said the dog put them both in a postion of no return.
Unless I saw something beyond the pale, in which case reporting him on RTF would be redundant because I would have already dealt with it more directly, I agree with those who said it is the owners responsibility to do due dilligence, watch said pro work, ask for references and inform myself. One last thing, we all have different personalities, as do our dogs and pros. A certain well respected trainer might be perfect for you and your dog, but could be a disaster for me and my dog, one size doesn't fit all. Do your homework.