You raise a central question while trying to help your friend.
I would tell him that many many millions of "healthy" people are living in utter misery and thousands of them commit suicide every day. Research has proved that incredibly, happiness is not correlated to health, money or age but it is an attitude and will to be happy under all circumstances.
We all need to learn to find peace and happiness in a creative way. PD can be crippling and limiting but it does not necessarily prevent us from being joyful and happy. One should have self examination to determine the sources of happiness before PD in order to adapt to the new situation with PD.
I wish your friend the courage and wisdom to live well with PD.