What happens if the speaker dies on the way to the conference? Suppose that two collaborators sent a paper to a conference which was very far away (say, from US to Australia). After the paper was accepted, one of the authors took the flight to the venue, and died on the way.

Conferences generally require someone to present their work for their paper to appear in proceedings. However, in this situation, it is nobody's fault. 

Is there a rule or a specific implementation for such cases?
It is up to the conference committee to decide in exceptional circumstances. This is probably all that can be said about this.