What does Billy's jealousy of Boyd's relationship with the girl prompt Billy to tell the girl?

Billy is jealous of Boyd's emerging relationship with the girl and Boyd's reaction is interesting in that he seems completely indifferent. Of course, it could be argued that Boyd, at fifteen and involved with his first crush, is so taken with the girl that he's willing to make stupid decisions such as walking away from his older brother in a foreign country. Billy's jealousy seems to prompt his announcement to the girl that there are things about Boyd that she doesn't know and that Billy is the only living person who does know them.