Had an experience back in the 70's on a short flight to Las Vegas flying an empty Lear 23 letting a friend fly. Descending from FL410 I was balancing the fuel in the clouds. When I looked at the instruments I saw a 45 degree bank increasing as we were approaching red line airspeed. I took over even though I had to lean sideways because initially I didn't believe the instruments until I saw all three attitude indicators said the same thing. He had a commercial pilots license and considerable time and it was day time. He was also 40 years old vs 20 in this case.
I do not think there is any way to tell if the captain was sleeping during this event but the 20 year old had no experience at all compared to my friend who lost control of our Lear.
Mexico cuts a lot of corners in how everything is done. We can assume initially that the FO was not legal to be a crewmember of a Lear for commercial operation. That part of the investigation they can prove. Unlikely anything else.