Despite the fact that your friend may not want people to know about their secret, it's best to tell your parents or a trusted adult about this. Dangerous secrets not only threaten your friend's life, but it can also affect you as well.
Not only is this healthy for you, but you'll also get enough practice and experience so that you'll be familiar with this area when respecting other people's boundaries. Decide what boundaries are important to you and respect them. If you can't respect yourself, then you can't respect others.