What surprise decision does Boyd make as he is healing?

Billy and the girl arrive to find Boyd continuing to improve. Over the coming days, Boyd and the girl spend a great deal of time together. One day Boyd tells Billy not to worry, that "I'm alright," and Billy counters that he isn't alright. Then one morning he wakes and finds that Boyd and the girl are gone. Billy notes that his life is less because he is now alone. He rides out without talking to anyone.