I scored a 21 on my Kaplan diagnostic before any studying, and then several months later scored a 21 on my real MCAT... However, I certainly did not study very much, 3 weeks at most and not very focused studying either. I annoyed the crap out of myself. It was all on me.
I retook the MCAT and scored a 25, and then one month after that one I actually retook it again and scored a 29.
I was pleased with the progress. However, with the cramming at the end I was still left with the feeling that i could have done better had I prepared better. I chalk up my first score to being an idiot who couldn't find the time to study. My second score was improved obviously, and I should note that this occured with my switch from Kaplan to the Exam Krackers books... In my opinion, Kaplan's books are simply a re-hash of most all the sciences we have taken as an undergrad. I felt that the exam kraker books were much more streamlined for successful studying for the MCAT. Not only that, but I actually found them much easier to read and study... I also got the Exam Krakers Audio Osmosis tapes from a buddy of mine and listened to them and found them quite entertaining and very helpful. The last few weeks before my second and third test I simply took practice test after practice test. I purchased the official ones online from the AAMC, and this was probably the thing that allowed me to improve my scores the most.
So I would suggest to your friend to calm down, make a study plan/schedule and stick with it. Not to try to learn every single detail of every single science, and to take many many many full length practice tests...