There's no correct answer, so I think you do the best you can with the information you have. Most of the time, people want their family to be notified in case of emergency so I would think that would be the default course of action. I'd be more troubled by a hospital or doctor making medical decisions for me without trying to contact family.

How would you handle it? Say you are in a park and you see somebody drop from an apparent heart attack. Do you start CPR or do you stand there wondering if maybe they have a do-not-resuscitate order on file somewhere?