One thing that I find helps many people is t write their feelings , ask your friend how he feels about writting Dad a letter, that way you dont get interupted and you can really think about your words before you write them.....just a sugestion, its worked for me in the past,
Not much to go on here...............how old is this friend of yours? What has their relationship been like in the past? Why does he think his father will not respond well? What "feelings" does he need to get out? I get the distinct impression there is something very specific he wants to talk to his father about..............
Without knowing the father/son dynamic at work here, I couldn't begin to hazard a guess as to how he should/could improve the relationship. The fact I'm NOT a psychologist or a psychiartrist rather limits my ability to offer you any real advice. Off the cuff I'd probably just tell your friend to sit down with his dad and speak his piece. But that might be the worst possible advice there is.
Perhaps if your friend could first talk about this with another trusted adult, maybe one who knows the father, he may gain some insights in how to proceed.
I hope there is someone out there who can offer you something more substantial than what I've given you.
I wish your friend the best.
Peace
Greenlydia