I don't know if there is a relationship between Stocks crashing to 50% and then having it's effect on Bitcoin to go down that low. On the contrary, if stocks crashes then those investors who have sold might have to find another assets to move, so it's either crypto, including Bitcoin and precious metal.
So I don't think this guy is right, but who knows, maybe there could be black swan events that will put a huge dent on Bitcoin. And if we go on history, every bear market, there is some unexpected event for us to go and experience lowest lows but I don't think that we can go that low though.
That could happen, but the chance is pretty low. Because, in reality, cryptocurrencies are still considered a risky market like stock, not a safe haven like gold. If the stock market falls 50% for any reason, money tends to flee risk assets altogether, rather than simply rotating from one to another.
The stock market has a huge impact on the economy. If it collapses and fall by more than 50%, that would be a disaster and an economic recession would be inevitable. Lets hope that does not happen.