There are a few times in life where you will become privy to information about a friend or family member either because they personally told you or you found out on your own, and you may be tasked with the decision of whether to expose the situation or them for that they are doing or to keep the secret.
1. They are cheating on a friend or family member.
This is a very tricky situation especially since one can assume you are very loyal to the friend/family that the person is cheating on. In many instances, people tend to confront the cheater and tell them that if they don't tell, you will which can work if they come around to actually doing it, but if that fails, you are in the uncomfortable position of having to decide whether you should mind your business or out the cheater. Be very very careful with this one. Though you are doing no wrong, and though you are tight as friends/family, you do not know what goes on behind closed doors, what the true situation is, and/or if that person suspects their SO of cheating and may become a target of either one or both of their anger/denial/lashing out. Also keep in mind, just because you expose the situation, doesn't mean either or will do anything about it.
Unless you have blatant evidence like a video or some pictures, come to your friend/family with more of just your concern about their partner vs. outright accusations. "I'm just a little worried that Jason isn't spending that much time with you and the kids anymore and that he comes home later and later--have you talked to him about it," or "I saw Erica last week with some guy at the grocery store, do you know him well?" Your friend/family may be in denial, may blame you for trying to break up their relationship/marriage, may end your relationship even if you're proven right. All sorts of things, so tread lightly whether you do or do not have concrete proof.
2. You discover a friend/family members drug/alcohol addiction.
Definitely talk to the person one on one first. Express your concerns for their health and their future. If you have no prior experience with dealing with addiction, you may want to see a counselor who can help you help them. If the addiction continues and spirals and the person is not getting any help/treatment, is engaging in riskier and riskier behavior, OR specifically a danger to others in the household especially younger children, expose them immediately. Get help for them, have them sent to treatment, and/or remove them from the house where they live because they need help and you have an obligation to protect others, especially children, from any danger that they may present. You may ultimately be unable to help them, but you should never enable or stand by and watch someone endanger themselves or endanger others by doing absolutely nothing.
3. Someone is suicidal
100%, expose them immediately to get help from family/friends/and medical/psychological professionals. This is not a time to be sensitive and not want to step on any toes or to mind your business. This is a life we are talking about and someone who is intent on ending it. You have to get over yourself and being polite and respectful and try to get them immediate help. Someone frequently bringing up suicide and acting differently from usual are major red flags that someone intends to go through with it---take that very very seriously.
4. Someone comes out as gay, bi, trans, etc.
There is a reason this person felt comfortable enough to tell you this. They trust you or that you won't actually go out and expose them. This is a very difficult thing to do, to come out, and it is not your place to tell the world for someone else without their express permission, that they are coming out even if you think your other friends/family would be cool with it. By attempting or actually outing someone, you are going against their trust in you and most likely killing your relationship, and you may even be putting them in danger. Don't do it.
5. Someone is doing serious illegal activity
With this one, keep in mind that simply having the knowledge about someone doing illegal activity and not exposing them could lead you to your own jail time. Hard one here too because you know by outing this person, most likely this will lead them to jail. If they have a family or some type of good life, you may feel as though you are the one destroying that dream or that life for them or breaking up a family, but you are not the one committing the crime and you cannot be held responsible for their personal actions. I would strongly advise you talk to this person and try to get them to stop whatever they are doing immediately for their sake and for their future, but if they refuse and it's too far gone, or again as in the addictions, they are putting you or other family/friends/or the community at large in danger physically or of going to jail yourself, then I think outing them is the best action.
6. Someone is being physically/sexually abused
For me personally, this would be a no brainer, to expose the abuser to the police, however, these cases can be a lot more complicated then that. A lot of times, especially if you find this out, vs. you are told about it, the person can be in denial about it, or you may risk putting the victim in more danger by exposing the perpetrator, or it may be hard to prove without evidence--meaning, it may continue on even after you expose the perp without solid proof. Very difficult situations.
If an adult victim is open to your help, help them document and record evidence for the police or get them to a shelter asap. I would not advise confronting the perp because you may be putting yourself or the victim at risk/danger especially if they have been known to be violent. Get the police and various other organizations involved. For children, you must expose. Often times, they do not have a voice, or other family/friends may refuse to believe them, but if you have the knowledge, report this immediately to an authority.