I know you're scared, but I'm going to tell the truth. This is just my experience only.
It is possible for a PALS to get to that point. It's not common. My wife lost every ability, including the ability to blink, and she could no longer move her eyeballs. Totally locked in with no ability to communicate. Fortunately, she died within 24 hours. I personally would not want to live that way. Neither did she.
With technology, there might be another way to communicate. Some people may want to live as long as possible.
It's important to have the PALS wishes in writing for the doctors to see, and let's hope the CALS and family agree to respect those wishes.
I have one general caution for all sons and daughters: The wife is the next of kin, and is usually the one that your dad will trust to carry out his last wishes. Interfering with their relationship, or trying to persuade dad to do what you want, can be detrimental to your relationship and emotionally painful for the PALS. I'm not saying you're doing this, it's just advice I give to everyone.
You must be brave. I'm glad you're trying to help. My attitude is this: Since we can't save them, our mission then becomes making each moment physically and emotionally painless and comfortable. Hopefully we can even give them a reason to smile from time to time. In the end, a painless death, without fear, is the most we can give our PALS.