Worst case he runs out of capital over a period of weeks.
He can't blow up in the way that you think - but he can have large drawdowns over a period of weeks.
Markets are eventually consistent scalable systems - and that is why we prefer them over central planning. In the medium term they prices things correctly, cheaply and efficiently (decade+).
In the short term however (sub-decade) - they can't price jack.
Markets are inefficient period - if they weren't, well then P=NP and you could just put your NP-hard problems into a market and get back cheap, quick, accurate results. Oh - wait - protein folding is actually harder than that.
There are 2 major ways to make money in the markets. Value-Growth and statistical arb (often high frequency). The former (Buffett) is highly concentrated bets on the future of business (I'm value - long TSLA/GOOG/Samsung). The latter is looking for thousands of small diversified statistically significant correlations above 50% (random guesses) and trading costs between securities/price movements over short time intervals (aka statistical ghosts in the data - RenTech/Shaw).
Both work. Both work well. And will continue to do so as long as markets exist.