You did the right thing. Some parents cannot believe things that their children do are not always the best, and you just telling them what you know is not wrong. Unfortunately you being the great person you seem to be are not as credible because you are not their son. You should tell your cousin that his family loves him through anything he does but that does not mean they will not be disappointed in him if he does do that sort of stuff. If he does not listen to you then you have tried his heart and if he does not care it will only be his problem. Just remember that you should never be upset for what you did. I know you probably don't want to do nothing, but the best way to learn is through mistakes and if he cannot realize that so many people have made this mistake before and regret it maybe he needs to know why.