[suicideneil]

Let me grab my crystal ball....

Could be any number of reasons- could be that the driveline got jammed and the motor stalled, pulling massive current that fried the windings and the esc at the same time- are you sure the motor is dead, has he tried it with another esc?

Could be that the motor was just too much for the esc and cooked it, the motor may be fine. Without testing it on another esc or inspecting it for blackend windings/ horrible burnt smell ( like a cigarette ashtray ) its impossible to say for sure...