Sounds like a pretty naive reaction on his part. If League of Legends was leaking credit/debit card information there'd be millions of cards lost and a massive class action lawsuit in the works. It wasn't League.
What was more likely to cause it:
- Not paying attention at a gas station pump, and having the bad luck of using the one pump with a skimmer. (The person who set up the skimmer can then sell that information to anyone looking to buy it, including someone in Mexico, or they can make a trip to Mexico then use it.)
- A virus on your computer that takes screenshots/logs keystrokes when you're entering your credit card into Amazon (prime days anyone?)
- A phishing email disguised as a bill that was mistaken as a real email.
- Checking your bank statements on public wifi.
- A data breach from some other company that had your debit card info.
Have your dad look into what actual leads to credit card thief before he causes more undo trouble for himself (or worse, has this happen again shortly after getting a new debit card).