If you know that someone isn't being faithful to his or her boyfriend or girlfriend, how can the person being cheated on be informed about it and ...

Unless it’s your sister or BFF, it would be much better for you to stay out of other people’s concerns .

I would be sensitive and tactful, but I feel like they should know...I was appalled when I learned how many people knew my ex wife was cheating (including my own father) and never said a word about it. No, they may not believe you (at first anyway) and they might even get angry with you, but the seed is planted in their mind and they will be more alert and observant as to what is going on.

My mother always said "what's done in the dark shall come to light". The cheated-on person will only believe it if they see for themselves. It will be revealed eventually.

One very strange thing that happens when you inform on a cheater is that your friend...(the person who is being cheated upon)...will usually hate you afterward. It's not logical, but it happens. Denial, followed by the wish to kill the messenger is a normal reaction. And never discount the possibility that they already know, and love the S.O. anyway.

I'm a person who is blunt and will tell u straight up.

It depends on if the cheater or the cheated is my friend. It also depends on whether the cheated is also cheating. I don't interfere in others' lives unless their behaviour causes harm to a lot of people or myself. And also the frequency of the adultery. If it's a one time thing, then it's not so serious

As for the telling part, you tell them how you think the person's cheating. Most people pick up on indicators that their partners are already cheating; like a lack of interest in their partners, sudden changed behaviour, which most people with basic intelligence can pick up on

If they know, it’s up to them what they do with the information. Maybe they don’t believe. Maybe they do but they’re saying they don’t because they don’t want you involved. Either way it’s out of your hands.

It’s someone else's relationship and you shouldn’t interfere.

Mind your own business not other peoples.

It’s their choice whether they believe it, or not. You can’t force someone to believe something. Now you are crossing boundaries into interference.

unless its like my best friend I don't say anything. my life has enough drama without having someone else drama!

They always mess up. Whether they are sloppy or a psycho informs the other person. It always comes out.

No issue ratting out a cheater

They will pick up on it soon enough.