I reckon that being filmed by his mate was a contributing factor. It made him show off and accelerating in a curve like that, it took him by surprise that the bike did not react as he intended. If he had known to counter steer correctly he could have corrected his trajectory before impact.
I think these things can happen also to young unexperienced dudes when they ride in a group and have to prove themselves without having the techniques to support their moves. He was a lucky bastard though, considering wearing a tee-shirt......as long as he learnt his lesson!