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.