Well, we know now Will did his best to destroy Coy's friendship with Boone, and he and his mother made sure Coy never dared to take it further than friendship. Also, it's pretty obvious Will killed Wes and Lee not just for their gold, but because he hated them for being a couple.
Is Boone a fool for having stuck around, for not taking Dan up on his offer ? Maybe, but I think he preferred the gold of his friendship and love for Coy, rather than pretending his liking for Dan could be a substitute, even if he thought he might have come to love Dan eventually.
But Coy was definitely a fool to pretend his brother had a heart of gold beneath his nasty behavior. I'm glad Boone confronted him with the falsity of that belief. Maybe Coy will finally learn to distinguish between the fool's gold of caring about the opinion of people who hate without reason, and the real gold of someone who cares about his happiness. But maybe it's too late.