At my school, it usually takes 3 yrs for someone to academically fail out of medical school (which very rarely happens, most people who do leave do so on their own accord). Our school has a maximum of 5 yrs to complete the MD program. If you fail 1st yr, you can re-take the 1st yr (you might stop taking classes all together once you fail, or you might continue with classes and see if you can pass any of the other ones (this is what most people do). So if someone fails first yr, and are re-taking it, they usually do not fail classes that they are re-taking because they know what's going to be on the tests, how to study, etc. So they pass 1st yr. Then they get to second yr. If they fail 2nd yr, then they are out. So the whole thing takes 3yrs. I've never heard of anyone at my school that failed, but I don't know too many upper classmen, but there are a couple of students who were in my class going through this process and I am wondering if they will be one of few who academically fail (they are on yr 3 this yr). So, unless you are worried about your rooomate just giving up on the whole med school thing, chances are she will pass but if she doesn't, she will probably take a few yrs to "fail out" if your school is anything like my school. Something else to think about would be that people who are academically struggling can sometimes stress other people out (I'm not susceptible to this "peer stress", but I have noticed that some of my classmates are). If you think that she might stress you out or if her behavior (her not studying when you both should be) would adversely effect your grade, that might be something to consider as well.
Oh, and I wouldn't assume that she will even academically struggle or fail out. MCAT and GPA are not perfectly correlated with how well someone does in med school. Probability-wise, she will most likely do fine and be walking across the stage to get her diploma along side with you 🙂 .