realising that everything is downhill from now on,
Self-fulfilling prophecy suggests he may end up more miserable than he is now.
No matter what age, if a person has the outlook that "everything is downhill from now on," that is probably the vibe they emit, and they may end up pretty lonely.
Obviously I believe like dyslexic; that the guy should pursue a relationship w/God or at least investigate Him.
And maybe your friend can volunteer in a position that helps others less fortunate than him. It can take his focus off things that depress him about his situation.
If he takes his focus off himself it should help...but if he can't, then he should try to focus on all of his blessings....down to the fact that he can see, walk, work, has a job, lives in a free country (I assume), isn't worrying about where to find his next meal, has a working toilet inside of his house w/toilet paper etc.