What should I do? I'm scared he'll do it again even though he said he wouldn't!?

Ok, so my boyfriend his bother and their friend went up to their barn and found 5 pots of marijuana growing on the window ledge. To see if it actually ws marijuana, they rolled it up and each took a puff... my boyfriend said he took a long drag on it and says he felt sick and light headed.. I told him not to do it again and he replied "I don't plan it, I won't..." and later they found out it was their dad who was growing it... I looked up to his dad and thought he was the coolest guy ever! But its scary cuz my boyfriend told me I cant tell anyone and thats why I'm not saying names,ages, or the where-abouts. Can you help me? What if he does it again? I'm really scared!