Well, I've heard of similar stories, of bank tellers robbing old people of their accounts hoping they wouldn't notice in order to fuel their cocaine addiction, so this case is not unique to gambling.
The teller might have noticed that most of the elderly people that are visiting the bank have some sights issues that's why should would have the mind to do this things.
Or memory issues. Also cognitive issues.
28K is a lot of money I would say according to google Bank teller salary is around 45K a year so it need at least 6 month to recover 28K
But that is if he spends nothing and uses his entire salary to pay back what he stole.