We are in agreement about his inexperience being the over-arching cause of the accident, but if you consider the Swiss Cheese Model of accident causation, having a breakdown of one safeguard (e.g., flying only in daylight) does not necessary cause the accident on it's own b/c the risk is mitigated by the other safeguards (e.g., being instrument rated, flying only in clear weather, etc).

For instance, they were flying at night but he had not planned on doing that-- they were supposed to depart hours earlier but they ran into very heavy traffic. Flying in daylight vs night would have increased his ability to fly with visual indicators, and decreased the likelihood that he would have experienced spatial disorientation. That alone might have prevented the crash altogether. Or, at the very least, it may have altered where they crash, which could have reduced the likelihood the crash would be fatal.

Alternatively, if he had been instrument rated, then he would have been able to use the instruments to orient himself instead of needing visual indicators, and then the fact that they were flying at night would not have been as much of a risk factor.