If crypto wasn't involved, do you think this employee would have gotten fired?
Except the bank has a laid down policy against using crypto or gambling with it, I think that it's unfair to fire the worker because he gambled with crypto. Perhaps there's more to the story but if it's just him gambling with crypto, especially not doing so during work hours, then there must be something wrong somewhere. Perhaps the bank prohibits their employees from engaging in gambling, not specifically crypto gambling alone, then it'll make some sense. There's no competition between cryptocurrency and fiat currencies so normally the issue of bankers holding or spending cryptocurrency shouldn't be an issue.
Firing someone is not the proper solution especially in this regard and without giving a direct notice is not advisable in any way. The bank employer may have done the thing he did by gambling during office hours, for which he could have been punished. But I think no one will agree to dismiss him from his job. But yes, if the bank has its own policy which is informed to that employee in advance, then there will not be much difficulty in dismissing him. But the real story may have changed differently which we do not know. I am sure that the banker was not punished for gambling, rather he was spending time on personal work during office hours, for which he will be punished. And since banking sites are a very important platform, employees should definitely work there with caution, otherwise the organization may face losses. He is guilty of wasting bank time, but there is no justification for convicting him for gambling or crypto gambling.