Boone knows his friend well. And yeah, Coy could never had kept that money. He was just concerned with protecting his brother, something he always tried to do, though the man never deserved his devotion.
Leaving Coy is the last thing that Boone wants to do, but he's been hurting a long time. He no longer needs to protect Coy from Will's baser influence, and he needs to leave a life he hates. It's not an easy choice, but the time has come. Yes, he will regret leaving, but he would also regret staying. Unrequited love is a pain like no other. Thanks, Terry!