Nopax, thanx, I'm not sure that would ever really be an issue. In very simple terms, a doctor has a duty of care to anyone he or she treats, with the level of that obligation being determined by an assumption that they are as reasonably competent as a hypothetical professional that does what they do at the level of experience that they have.
In other words, if the treatment that they give to a passenger is questioned at a later date, a court would look at how experienced the doctor was, what it is he or she had to do, what materials he or she had to work with, and any other circumstances particular to the case. The court would then attempt to compare what that doctor did, in that situation, with what a presumed hypothetical doctor with the same experience would have done in a similar situation.
I'm not sure there is an obligation (legal at least) on doctors to assist passengers in distress, but I imagine that the doctor's professional body might have something to say if they didn't, and personally I think that there should, of course, at least be a moral obligation to assist. For what it's worth there is absolutely no obligation on anyone to come to someone's aid - if you're walking past a river and see someone drowning, there is not obligation on you to dive in and rescue them, even if you are an olympic swimmer or if the water is only knee high. Beware, however, if you do. If you attempt to rescue someone but are so cack-handed at it that you in fact make things worse, there is every possibility of you being sued for negilgence...
Funny old world, eh?