So you find out that a friend has started hanging out with someone that you know is bad news. You know for a fact due to past experience with him / her that this person has the morals of an alley cat and that your friend will end up used and or hurt. Your friend is actually a principled person but cannot see the bad in their new association. You know that if you speak up you might lose the friendship, but sitting idly by while your friend puts him / herself at risk is something that feels alien to you.
WWYD? Speak up and risk the friendship? Stay quiet but take some distance? Warn your friend anonymously?