How old is your friend? If puberty is very late, then there could be a problem with the endocrine system and a proper workup is necessary. For example, a pituitary tumour can cause delayed sexual development. If she has menses already and it is just a matter of smaller breasts, then it may not be a medical matter. If she is starting puberty slightly later it might be a good thing, cos' when she starts she will probbaly end up taller than her friends.