My partner's medical insurance covers her worldwide for situations like this where she's acting as a 'good Samaritan'. I imagine there's various laws around the world which may prevent a doctors from being liable, but given global travel it's probably better to have insurance rather than trying to read up on the law from various countries you are holidaying in.

The reason she gave for not accepting an upgrade is that her medical insurance wouldn't cover her if she was doing paid work for the airline. She was told (by other doctors, so make of it what you will) that the upgrade could be viewed as payment for her work onboard and thus her insurance wouldn't be valid.