People have died in amateur MMA shows before. One in North Carolina I think it was like 2 years ago and one in Russia about 7 years ago. The kid in NC had a heart problem though and they didn't med test him before the fight.
I mean he is an athlete, he can be trained, but by no means does hockey fighting transfer to MMA success. He can have good hands and motion along with conditioning which will all help but to think he can just drop in and win is foolish