Question is if it’s reasonable for the father to guess letting the kid use the account would let the kid do this kind of charge. I would say no and put the blame on google.

As a parent, I would say yes. Anyone who has kids know they spend any money you give them access to. I daughter spent $100 on snacks in a week because it on her school swipe account. Kids are just like this. My kid has her own kids google account where all purchases have to be approved explicitly by me, the parent account. The dad could have done that, but he didn't. This is literally why kids accounts exist. There are many horror stories about kids spending their parents money going bad decades. Every parent has a story. There's no way the dad couldn't have considered this as a possibility. This is setting aside that he shouldn't have mixed his work cards with his personal accounts.