I was in the same camp as your bro, still am. I have no problems with open endings or things not ending happily, but this is one of the times like I felt like it was the author not really knowing how to finish it. It almost felt like he did it with some malice, as if he had something against the series or its fans, but that I don't know about. All I do know is I'm supposed to say "... but later I realized it was the perfect ending, and couldn't have ended any other way". That seems to be the agreed upon defense of the ending among those who either liked it or came to accept it because they had no choice.
There is a reason he's in the loop, as best as I understand it, though I can't remember if they ever actually came right out and said it or if I read it in the Concordance or something. But it's not like Roland has been condemned to some sort of purgatory and is just doomed to an eternity of questing for the Tower. He just won't get to enter the room at the top of the tower until he gets his quest "right". Which mistakes he made and what must be done to get it right are up for debate. Apparently the Horn Of Eld is a piece in the puzzle, but it's more than that, his actions and treatment of others and his ka-tet. Maybe a little less innocent blood spilled along the way, maybe different outcomes for his companions, a change in his attitude that reaching the Tower is more important than any of these people.
My opinion has always been that maybe King should have written the cycle where he got it right instead of a dress rehearsal.