I see lot's of downsides to believing, mostly because it causes people to forsake they one life they know they definitively have for one they
may have after death. Belief makes people not eat bacon, or a good steak on Friday evening, or forgo alcohol or shellfish, explore their sexuality, disown gay family members and so many other things. I have a friend who walked away from the perfect woman for him, it was unbelievable how great they were together, just because she wasn't Catholic. That makes me sad and his life has never been as happy but because of his belief he chose to gamble on an unknowable afterlife while throwing away a happy life
I try to separate whether or not there are downsides from base belief. For me my belief system would continually get washed away by single events or perceived injustices. I would ignore the obvious due to my own issues. I think it is what Jesus was talking about when he described the house built on sand. Basing ones belief on how THEY would like to live their life (as I did for almost a decade) is a choice that has consequences too, and in my experience some very self destructive ones. I never could find that happy life in self indulgence. There was always something I wanted and couldn't get. I could never find real contentment. Aging was depressing.
Like I said earlier. I had to come to some belief in a God first before I came to believe what I do about the nature of God. Lots of people throughout history believe/d that there is a God/Gods, but not a kind and gracious one.
When I believed in a God, but believed he was cruel and unkind, I rebelled and did not care.Tried to prove it wrong, and was humbled. I did not take it very well and lost my belief altogether.
When I did not believe, my personal choices where more selfish, self indulgent, destructive to myself and others.
I believe that God is kind, gracious and wants the best for me. Actually created me with a purpose to do good, and has an ultimate plan for me. That makes a great deal of difference. My happiness in this life is not that important to me, but I have never been happier.
As I said before none of these is necessarily tied to belief. One can be altruistic without belief for the good of mankind. However, the fact that people can make decisions that are not based on selfishness is one of the hundreds of reasons I do believe, but most of my most substantive proofs have come afterward testing what God promised. IMO it does not line up very well with Darwinian thought.
IMO your friend is sad because he wants something that he cannot have. Guess what? We all could be sad, and are sometimes sad about that. It is much healthier to be content with what I do have and look forward to better times. It is necessary to take ones lumps from what I see as past mistakes and move on. He can make the same choice whether he believes in God or not, but that is a constant theme in the Bible. It would be much better than drowning his sorrows in a beer (a choice I used to make).
Belief does offer a psychologically better way.