If your parents still have the capacity to make their own decisions, then they can make those decisions. An AHCD doesn’t come into effect unless they’re incapacitated, and it will stop being relevant if they’re no longer incapacitated, such as if they wake from a coma. If your parent is incapacitated, the agent listed in their directive can make decisions. In most circumstances, you can only decide for them if you’re their medical proxy.