I don't know if the story you just told is true and if you know any such billionaire, but first, in what currency was this person a billionaire, one can be a billionaire in certain currencies, but still not be considered so wealthy if we compare it with currencies like the U.S. dollar, GBP, Euro, etc.

That being said billionaires do not keep their money in fiat, the keep it in different investments/assets, they diversify, and most of these assets will be priced in USD, the chances that all of their assets and businesses will fail or collapse all at once is very slim and so it makes it hard for what you say to happen.