DO stands for doctor of osteopathy. I would direct your cousin to this site and have her read some of the stickies in the DO forum.
The consensus regarding the major difference between DO and MD physicians seems to lie within OMM (osteopathic manipulative medicine, I believe)-- DO's learn this during medical school and MD's don't. DO's tend to take a more holistic view of the person than their allopathic counterparts.
DO's have their own residencies but can compete with MD's for allopathic residencies, and some programs would require that the osteopathic candidates sit for the USMLE as well as the COMLEX.
No degree is "better" than the other, choosing MD over DO or vice versa is simply a matter of individual preference. Sometimes, if someone is interested in entering certain specialties, they choose the allopathic route over the osteopathic route, or vice versa.
Generally the average mcat scores for DO schools tend to be a few points lower than allopathic (MD) schools. DO schools seem to look more at the individual and are more likely to look past the numbers, but choosing a school based on the fact that it's mcat scores are lower is generally not the way to go.
Tell your friend congratulations on her high school graduation and tell her to enjoy college. She has more than enough time to do the research and decide which path would be best for her if she decides to persue medicine.
