Ok, people, where's the positivity? You are doing a poor job at comforting and are moreso solidifying the potentially untrue generalization that this is a terrible death sentence. The person will continue to be the person you know and love. Their degeneration will be hard, but it can be controlled and slowed with the correct drugs. It's not a fun situation to be in, but sometimes having something like this can be a blessing in disguise, allowing people to do what they couldn't before.
OP, I deal with multiple Parkinson's patients biweekly. They are some of the most interesting, sweetest, and hardest working people I know. Their determination is amazing and it gives you a greater appreciation for them. Enjoy the time you have with the loved one, but do not pity them. That is the worst possible thing you can do. Be there for support and just be there to talk.