What broke my heart was how sad his father was when I told him, he said he trusted the son the most.
At eighteen, the boy is an adult and would take full responsibility for his actions and inactions. Parents indeed need to closely monitor their children's activities online, but at that age, it will be very difficult to watch over that boy. This child simply chose to be a criminal, maybe he was influenced by friends or other agents of socialisation.
For me, if a child is still under my roof, I would closely monitor their income and expenditure. If he is spending more than I think he should, it will be the right time to investigate his sources of income. It is also important to always investigate the source of your children's income. Don't rely on what he or she told you.
Like from the title of the thread and the beginning of the story, you can't really monitor someone or know about what they have with Bitcoin because this is personal. Whatever anyone tells you about their engagement with Bitcoin is what you will believe. It is easy to monitor children when it comes to a source of income that is physical, but if it is something online, it is not easy to know everything in detail because everything online is designed to be private, which people must keep to themselves. As a parent, you can't even monitor the crypto journey of your children because this is personal, and it is their responsibility to be in charge. Unless they want you to be involved, then they can make room for it.