Those of you saying she only got in because of her privilege are plain wrong. was it a factor? Yes. Of course.
However there are very clear bars that you have to meet in medical school, including passing nationally administered exams that require months of studying - you literally have time built into your schedule to only study for Step 1 and Step 2. Very few people fail out of medical school because medical schools are surprisingly good at only accepting individuals who are going to succeed. The average GPA for a MD student is 3.8+ and the average MCAT is in the upper 80 percentile.
Stanford, one of the top medical schools in the world, would not have accepted Cain, no matter her name or connections, without thinking she was going to succeed. Doesn't mean those didn't help her, but this isn't undergrad. Med school admissions do not mess around because they recognize the resources they will have to pour into students and the real responsibility those individuals will have a few short years later.
Also individuals should give her a break for decisions she made at 16. What kind of decisions did you make then that you wished you had been more mature about? What did you not stay committed to that you wished you had?