If your friend is in medical school, I would say that he probably should go ahead and take the exams! Who knows? He may even pull up his grades to the point where he passes some of the classes he is having trouble with. If he takes the exams and still doesn't pass, at least he will have an idea of what he needs to focus on so that he can do better next year.
Most schools have a medical student evaluation committee that evaluates students in academic trouble and makes recommendations for remediation and/or dismissal. Taking these exams may make a world of difference as to what they decide to do about him. If he only fails one class, it is likely that they will make him just do one of the summer make-up courses. I think that by taking the exams, it will show that he is interested in the program and is trying to do better. The committee is likely to look on him more favorably than they would if he just gives up at this point.
I would also suggest that he speak with his Deans and faculty for their recommendations about the exams.
