I don’t think boy B planned it at all, but he did have his chance to tell what had happened when everyone was looking for Ana. To come forward and tell, as a voluntary witness. Supposedly you don’t expect the murder (but probably expect the rape, or something not legit anyhow; bad, but still, not murder). But then you watch your friend kill a person. Would you not run home sobbing and call the police?
So yes, boy B had the chance, and lost it.
Exactly
You have to take the Court into your confidence if you want the benefit of the doubt.