Headspace on a M16 Series Weapon?

In what way did the bolt "come apart"? Did it shear one of the lugs?

No, it split where the cam pin goes inside of the bolt itself. it split up and down as the bolt sits in the bolt carrier with the weapon upright in the firing position.

From what I understand, all rifles and semi-auto pistols have a head space that needs to be set either at the factory or by a smith (an ambitious do-it-yourselfer could probably do it too) in order for the round to be seated properly. Too little head space and the bolt will impact the casing too hard and may cause damage to the weapon in extreme cases. Too much head space and the round may not fire or the bolt may not lock properly.

I'm guessing that your friend did not check the head space after installing the barrel and it was too tight. Repeated slamming of the bolt probably caused the fracture.

Or the bolt was defective to start with.

Pictures of the broken part would help though.

Yeah I am going to get some from him, I just had a brain fart after we fired on the range and didn't take any. I took plenty (actually my significant other did - thank goodness she loves to shoot but doesn't have a well developed trigger finger muscle ).