Thank you very much again. I used the term "MD" loosely - I don't quite know what it is. He finished 6 years, which makes him a "doctor" in Russia. This isn't a phd though. Seriously, thank you.
Here, a PhD is an advanced degree that you pursue after your Bachelors that trains someone to do research (and, arguably, teach) in a particular field. One can get a PhD in essentially any subject: Sociology, Psychology, Microbiology, Biophysics, Physics, Chemistry, Anthropology, etc. Though one can do medical research with a PhD (ex, research the causes of cancer), it is an entirely separate degree and path; it is not a medical degree of any kind.
Here, an MD is our medical degree -- the one that makes someone a doctor. It is completed in four years after one receives their Bachelors degree (which can be in any subject). It trains someone to practice medicine generally and prepares them for further, required training. This training is called a Residency program and is the point where one specializes. This training is needed to practice medicine in the US.
(Sometimes, someone will have both an MD and a PhD. Usually, people who pursue both degrees want to be able to both practice medicine and do research).
As your friend already has a medical degree -- what is considered to be an international equivalent to our MD -- to practice in the US he must complete a Residency program in a particular specialty, ex Pediatrics, Family Medicine, Surgery, etc. To do this, he must first apply for Residency.
Before applying for Residency, he must take the USMLEs. These are the licensing examinations given to ensure competency in licensed doctors. There are three of them (Step 1, 2 and 3). They must all be passed and done well on. Studying is usually required.
If your friend, on the other hand, just wants to come here to do research, not practice, he would get a PhD in whatever field he wanted to do research in. To do this, he would have to take a test called the GREs, the TOFEL and apply to each particular school.