Would you ever deny a friend something they wanted to do that you knew wasn’t good for them?

You aren’t responsible for your friends.

> Would you ever deny a friend something they wanted to do that you knew wasn’t good for them?

Ahm...yes? A friend isn't characterized by how much they agree with you. It's perfectly valid to say no.

I would definitely try to stop my friend from doing something I see as unhealthy or dangerous.

As others have said - absolutely, yes. Responsibility has very little to do with it, it's all about wanting the best outcome for them. And that should apply to both your children and your friends.