In some parts of the world $50k means nothing. So whether you talk about your winnings or not make no difference. If that Nigerian man still lives in Africa, then yes he should keep his mouth shut because African countries are not the richest countries in the world and the crime rate in those countries are pretty terrible. In the US though, many people make $50k in 6-7 months it is not a big deal at all. Nobody will care or murder you for that money.
What have had made the man to let the man to talk about is victory could be too much excitement, who knows how long he has been playing gambling but couldn't not win. Sometimes too much excitement can make one to over react, expecially when the money is too big for some people to afford.