<...> the condition is that if he is able to swim across the river, he gets about $0.3 which is equivalent to about 300 naira.
I wouldn't call that a bet, given how low the amount was. It's simply a silly thing, like many of us did when we were young, which unfortunately went wrong. I remember betting beers or dinners, which in the end never got paid, to do silly things—though luckily, none of them had a tragic outcome.
This is so sad. It's almost similar to the news I came across about a teenager who injected himself with butterfly resins because he was participating in an online challenge and it cost him his dear life.
We are in an era where people do crazy things whether for fun or money but they forget one important thing, that life is priceless and we've got no spare. So no amount of money should warrant anyone putting his/her life at risk.
I'm not sure whether more silly things are done nowadays or if we just hear about them more because of the internet. However, without more data, I'd bet that more silly things are done now than 50 years ago or more.