He said they parted ways, rather than had a falling out, there's no indicating that there would be conflict, OP correct me if that's the case.
A lot of people turned up to my mothers funeral, many of them didn't get along with each other, many of them didn't know my mother very well but in an environment like that you set your differences aside, pay your respects and be there for those who need someone to be there for them. That's what I remember most about that day. Losing a parent is tough and unless the OP has some serious issues with drug taking hipsters that he can't set aside I'd hope he could attend in a positive way.
I've always found it difficult to say "I'm sorry" or variations thereof, but then I realised it just isn't about me, what I find difficult, my baggage or my experience, it's about letting those who are grieving know they're not alone, that the deceased will be remembered. That phrase "I'm sorry" seems so utterly meaningless and hollow but it's just what we've cobbled together as a verbal substitute for something I don't think we can really put into words.
The state of the friendship is kind of irrelevant, attend, pay your respects, remember his father and if you think you can be of any comfort to those grieving do what you feel you can.
Eh, I guess I just don't really see the point of all that.
And yes, I've lost people close to me, my grandfather and my uncle that I was very close to. I didn't go to either funeral.