the warning

Quite often you get more appreciation for helping someone when they are already in trouble than for trying to help them keep out of trouble before they are in it.  I've found that warnings, no matter how helpfully intended and no matter how politely or lovingly presented, are often disregarded, or the person may even get angry.  Where-as, heartless as it seems, if you just sit by quietly, and then help if they do end up getting in trouble, they're pleased as punch with you. Of course if you're really close to the person, like family or a best friend, you have to warn them anyway and just deal with it cuz you love them.  But I think for acquaintances or strangers, sometimes it's better to just let them go on their way.