Thinking about this, I have mixed feelings. On the one hand, I think your concerns about the difficulty of maintaining a high GPA at CMU and obtaining superior grades in the prerequisite classes are legitimate given the high stats of its student body. CMU is absolutely stacked with high achieving STEM students. Per the 2018-2019 Common Data Set for CMU, the 25th percentile in math was 750 and the 75th percentile was 800. This means the average CMU student (in that reported class) is walking around with a 775 SAT math score! That is incredibly high! Compare that to the average math score of a BC student of 709 per BC’s 2018-2019 Fact Book -which is very good but not at that kind of level. Which group would you rather your daughter compete for an A in organic chemistry or in other prerequisite classes? I acknowledge high math scores do not necessarily equate to corresponding ability in other science classes but I would expect a high correlation. Many of the kids that in the stratosphere in terms of math scores are also top performers in science. Assuming your daughter has the average stats of a CMU freshman, while nothing is guaranteed I would bet on her performing in the top quarter of BC students before being average or below average. I would not take that same bet with CMU. Advantage to attending BC when looked at through this lense.
On the other hand, the statistic that only a small percentage of pre-meds actually matriculate to medical school should be paid attention to. If your daughter for any reason does not pursue medical school but pivots to another STEM related interest I would argue she would be better off at CMU due to the obvious strengths and focus of school.
It goes without saying, none if this matters if fit has not been factored in. No kid is going to well in the long term where they are not happy or if they feel they did not get enough input in the final selection. The demographics are also markedly different at both schools. Again, this could influence fit depending on the background of the student.
I hope none of the advice in the first paragraph is construed as a back handed slap at BC or is any insinuation that it is an easy school. It is not. I hold the school in very high regard. I have a brother who attended BC and went on to attend an Ivy league medical school from there. His journey was not a cake walk. If you do well there you can go anywhere from BC. Good luck to your daughter in whatever choice she makes!