Although the betting shop has a poor level of security, such as one of them does not have CCTV at all to at least be used as something to find out about the appearance or characteristics of the person, but in my opinion the incident could have been thwarted or less likely to happen if the cashier really put high alertness and caution by asking the gambler for money every time he wanted to start the game.
So I think the core of the problem is because the cashier is too careless in terms of trusting others, that is a big mistake because we never know whether other people have bad intentions or not, remember that crime does not always happen because of intent but because of opportunity, and yes in the end maybe as you said that most likely the cashier will be fired from his job.
It's just simple stupidity to allow someone to continue betting on credit without payment because I can't seem to rap my around the logic of allow the gambler continue playing except maybe the cashier also is a gambler and believe luck will maybe reach the gambler and then he would pay off but unknowingly to him that sometimes gambling can be really messed up and you can continue chasing your loses till God knows when and still you won't get that amount you are looking for.
Honestly I can't say that the cashier was too kind to let the gambler gamble without first collecting the payment, but what is certain is that he was very careless in terms of trust, and on the other hand I think regardless of whether the cashier is a gambler or not I think it has nothing to do with this problem, because he should have acted professionally, in the sense of understanding that work is a job that is covered by many rules and regulations and gambling is something else, meaning it is not something that can be used as a defense.
So maybe in my opinion the cashier was not professional and careful in carrying out his work so that he did not apply caution and vigilance, I will repeat the idea that I have conveyed before that crime does not always occur because of intent but because of opportunity.