One of the two was diagnosed with sleep apnea after this incident. That explains why he would have fallen asleep; it is a severe sleep disorder that causes extreme exhaustion, and many fatal accidents occur with drivers suffering from this disorder.
I don't know if FAA rules allow someone with sleep apnea to fly aircraft. If not, this pilot may be considered disabled due to this condition. It is treatable, with a device called a CPAP, that forces pressurized air into the patient, in order to maintain blood oxygen levels at safe levels while sleeping.
The second pilot apparently does not have any medical condition that caused him to nod off.