"Am I my brother's keeper?"
Yes. Yes, you are. We have an obligation to hold our friends and neighbors to a high moral and ethical standard, and to protect each other.
What kind of friend are you? If you hear your friend wants to jump off a cliff, are you the type of friend that: 1) hands them a map to the Grand Canyon, 2) the kind that walks to the cliff with them and cheers them on as they jump, 3) the kind that says its okay to jump if the cliff is of a certain height, 4) says "that is a bad idea" then does nothing more, or 5) grabs them and says "you have a serious problem and we are going to talk and get you some help"?
If you are anything other than than type five, you are not a very good friend. If you are any of the first three, you are not a friend at all, but an enemy. You do not have that person's best interest in mind.
Be your brother and sister's keeper. Don't fail your friends. You are not being nosy, gossipy, or judgmental if you are trying to stop your friends from making bad mistakes or warning them about the harm they are doing to themselves or others.
Gossip is telling "dirt" about people you know to anyone OTHER than the person the "dirt" is about. Being judgemental is when you accuse someone in such a way as to belittle them.
It is not nosy to address gossip with the one whom the gossip is about.
You are not being judgemental when you express concern to a friend that their decisions or actions are going to cause grief, pain, or harm.