Glad the nephew is reletively well... all things considered.
Sounds to me that the frame dmg is more than likely from hitting something, with the forks sheering off with the front wheel as well, not something that is likely to happen in the case of the frame letting go I dont think. I got a feeling your nephew may not have slowed down at all and the bike hit something hard at quite a reasonable speed. Like's been said, you'll have to see what his side of the story is before you plough in at full steam.