Hi Peg, hope you are keeping a bit better than I am at present!

Tell your friend that most people have a distraction which makes enjoying yourself more difficult. (Notice I did not say impossible!!).
For example you may be pitifully poor, and lack nutrition, secure housing
entertainment etc. You may be basttling cancer, or any one of a number of serious diseases. You might be getting used to no right leg after a spell in Afganistan.
You could go on making up dozens more such situations, until your friend finally sees hiss suffering is not unique, indeed there are many worse situtions.
Then point out it is up to him to manage his own life, you only get one. See if he can find a hobby or find how he can help others, to make good use of the life he has left. See my defunct silverware hobby. I visit the PD research team in London every 2 months. I help where I can, but it is very interesting. Get him to recognise that everyone has some misfortune to navigate through. Some a bit worse than others.
The trick is not to waste your remainlng years whining. Do something which is useful, and keeps him happy

Best wishes
Ron