Take a look at this:
http://www.healthplanusa.net/DGInFlightMedEmergency.htm
Legal liability for the doctor. British Airways says it indemnifies doctors who answer emergency calls from malpractice suits that may be brought by litigious passengers.
Currently, all 50 states have some form of Good Samaritan legislation. But which legislation covers a physician who is flying? The Aviation Medical Assistance Act, passed by Congress in 1998, "...limits non-employee passenger liability for providing assistance during an in-flight medical event unless the assistance is grossly negligent, or is willful misconduct." To date, no physician has been sued successfully for rendering assistance during in-flight emergencies.
I'm not a lawyer either, but usually, the law which applies to a negligent act is based on the place where the act occurred--since an aircraft is sort of considered to be a flying exclave of the country where it is registered, that would imply that the law would be the law of the aircraft's country of registration. So there are all sorts of laws that could theoretically apply...
Ethically, the answer to your question depends on how you define the duties of a doctor. How do you think it would sound if someone died on an airplane, and there was a doctor on board who never bothered to help?