Please clarify.
Is he really coming for an observership? i.e., is his real intent to complete observership and go back?
If that is true this is the course he should take . Show proof of his current practice/employment in India, also show assets bank statemtnts et., Convince the visa officer that he is not going to stay back here. For that he can mention that he did not take the USMLE 1 and 2 which he could have done in India, because he has no intention of emigrating. Also he needs to show that the experience he will gain in the observership is not readily/easily available to him in India. A letter from a local hospital stating that the skills he acquires during his observership will be of great use to the institution and local patients upon his return would be very useful.
These have helped others in the past. Also he needs to come on a B-1 visa.
What specialty and what kind of specific observership is this. if you give me details I can help better.