Muscles or spine, the most common reason people injure either is improper lifting. And the fact that squats are good for you and almost universally used by professional trainers can hardly be called "gym mythology." At least, not as easily as one man's opinion can be called "gym mythology." And it really doesn't matter what the guy's done, he still can only have one man's opinion.
When you talk to a lot of different professional trainers and physical therapists about training and exercise (I have) you get a broader perspective. For instance, I've had trainers tell me to stay away from bench presses because they are a good way to injure your shoulders. Which are by the way, one of the easiest joints in your body to injure. And there are much safer ways to strengthen your upper body. Yet, your friend doesn't seem to have a problem doing them.
