Your friend is lying; not a doctor in the world would have told her that. And how does a 15 year old get to talk to 'plenty of doctors'?
Breast cancer, like all other cancers, happens when normal cells change so that they grow in an uncontrolled way. This uncontrolled growth causes a tumour to form. No amount of hitting, biting, pinching, twisting, squeezing can possibly trigger this process. Nor can any kind of bra, or any of the other nonsense teenage girls scare each other silly with.
Cancer is NOT caused by external trauma.
By the way, for future reference: a 15 year old with breast cancer would make news across the world. A girl that age is more than twice as likely to be struck by lightning. A woman in her 40s with breast cancer is considered unusually young; I was classed as a 'younger woman' when I was diagnosed with it because I was only 50.