That's true. Training, and sustained careers in elite sports, are hard on the body, and the damage can manifest itself at a young age if you're doing a lot of repetitions of something that puts a lot of stress on your body. Olympia mentioned Tara Lipinski, and you have high school kids getting Tommy John surgery. That's serious stuff.
The question is how far and how long athletes should continue to push themselves. I don't have an answer to that, but if one is to achieve anything in sport, sometimes you do have to push yourself to the limit of your endurance, and it's not always easy to tell when your cross the line from the limits to past them. From other accounts I've read, I suspect my friend is right when she says that denial is a potent aphrodisiac. Ignoring pain or pushing through it is something many athletes are taught to do - no pain, no gain, etc.; we just saw Robert Griffin III suffer a serious injury that might have been preventable, while conversely, the Washington Nationals last season had the good sense and courage to shut down Stephen Strassburg to protect his surgically repaired arm. And there was plenty of criticism of that decision. How far do we want athletes to go in order to entertain us? But OTOH, is it our place to tell them what decisions they should make about their lives?
Again, no answers to any of this.