The problem is such look like a safe bet when in fact not... I have seen people lose on a 1.005 odd bet which looked like a sure bet but turned out to be a high risk bet that will lead to losing and the unfortunate thing about such wagers is that a player needs to put in alot of money to have a small profit but in the end its all burst! Btw I think bookies actually make more money on such bets because of our greed, it's a hard lesson learnt and can never be too sure of an outcome.
It seems some gamblers forget that even bets which are incredibly likely to happen and which may seem as an easy way to make money are still not a sure thing, however the mistake was not really to make a bet in such a likely bet but to bet such a huge amount of money, doing such a thing is never recommended and this massive loss is a good warning sign so people that could be thinking about doing the same thing realize that it is never going to work.
Most likely that what happened here, or he really has that capability to make that huge amount of bet thinking that because of high probability to win the chance of losing is next to impossible, but likewise shit can happen and it does with his bet, the huge lead before he left his bet is really something that you will think that when checking it back you will just cashing out your winning.
But the outcome is the opposite of what he would expect and nothing he can do now, that experienced will serve as the basis to everyone to always know how to judge the amount that you can afford to lose before placing your bet.