SHIH-FU:

I would ask to whom are you being compassionate? If the person is in a coma, then he or she is not aware of anything. If the self has left the body that is being kept alive, then there is also no issue. That leaves compassion toward the living.

It is not certain in those cases if there is still a self present in the body. It is possible that the self left, that it has even taken a new birth, or that it is still holding onto the body. How can one be certain? Even if a medium tells you the self is gone, how can you be certain the medium is correct? If the spirit of the person tells you directly that the body is lifeless, how can you be certain that it is really and truly the spirit of the person? It could be your imagination. It could be a ghost or deity playing with you. In the end, however, you are left with a decision to make. Buddhism does not dictate what you can and cannot do. It speaks of causes and consequences. You are free to act. If you feel it is in the best interest of everyone to pull the plug and let the body expire, and you follow through on your decision, then there will be consequences for your action. The consequences may be good or bad, light or heavy. In any case, you will not know when the consequence arises, just as you are now living the consequences for things done in the past. Your life now is the consequence of your previous actions. Do you know what your present consequences derive from? What you do now will sow the seeds for future consequences. There is no merit counter in the sky who is judging you, who is going to forgive you or punish you. It's your move.