Even if you are a foreigner, in addition to government regulations, you also need to comply with the regulations of the bank/company where you work. You are their employee and you need to follow their rules, and with the anti-gambling culture in Korea, it is no surprise that banks would prohibit gambling.
Whether he used the bank's computer or his phone to gamble, it was the same, since he gambled during working hours, his superiors had the right to handle it according to company regulations.
Stealing money from a bank/company is never easy but for a gambling addict with no money to gamble anything is possible.
That is totally not banks business what I am doing with in my phone. That is a violation of privacy. How would they even find out that I am gambling on my phone? By looking on the screen? If that is ok, then it would be ok to browse my chats and phone call list? Absolutely not. An employee is not a slave that has no right to spend a minute to take a rest. Then everyone should immediately get fired as soon as they went to a toilet to get a cup of water.
For addicted anything is possible, but from my experience, to steal from bank, where cash is monitored by dozens of cameras, and UI wont allow to make a transaction to a random account without receiving a permission, is close to impossible. Imo the maximum what addicted can steal is money from his colleagues.
The thing is, banks hire and pay you to work for them, they don't hire and pay you to gamble at work and during work hours. If you want to gamble or do anything on your phone, do it at home during your break or anywhere else, not at work and during work hours.
If you were a boss, would you be happy to spend money to hire employees but they do not work hard and only spend time gambling during working hours?
Don't compare going to the toilet, drinking water... with gambling during work hours, that's pretty lame.
Whether it is customer money or co-workers money, it is theft and is taboo in the company. That is why every company has its own rules and regulations that employees need to follow, we need to take preventive measures instead of letting things happen and then trying to solve them.