What type of visa does my friend need?

Yes, he can try to come on a visitor's visa if he can get that. Once in the US, if he is accepted into a program, the school can help with the change of status. Sometimes the change of status is not approved within the US, though, and the person still has to go back to his home country to apply for a student visa but the student visa may be denied at the Consulate.

If your friend intends to come to the US to the US to study, he should seek an F-1 student visa. Alternatively, he may come to the US on a tourist visa for a 6 months period. During that time, he may look around the various academic institutions of his liking and then apply for admission to the one of his choice. Upon acceptance he can change status from B-1 to F-1 student. The caution is that if he enters US as a tourist, and a border inspector finds that his real reason for coming to the US is ultimately to pursue a course of study, he may be refused admission on the belief that he misrepresenting his real intents.

The way you described is the correct one: your friend should ask the U.S. Embassy for a B visa so he could come visit the schools and select one where he would want to become a student. Every school that accepts foreign students has a special staffer, called Designated School Official, who prepares paperwork necessary to obtain a student visa - or to change status in the U.S., without going back home. Filling out an application for change of status (Form I-539) is not a very difficult task, at least, for a prospective medical student. The form and the instructions are at uscis.gov/forms.