Stress on a bone= Wolff's Law of Bone Density. A healthy bone adapts to the loads placed upon it. Heavy loads= A shit load of bone growth.
Heavy squatting, deadlifting, and even benching to a certain extent cause increases in releases of testosterone and GH. More GH means more bone growth. Thats why midgets are midgets... because they have low GH... and they don't squat heavy enough.
I am willing to bet your bodybuilding friend is weak and has never read a book without pop-up pictures in it. All the bullshit about lifting heavy weights and growth being stunted is just that, bullshit. If your form sucks and you try to lift weights that are too heavy for you and you break your leg because you are stupid, then that is a different story. A program focused on good form early on and not lifting like a flex magazine dumbass will keep you from breaking bones while you lift weights.