Should you inform someone their partner is cheating on them?
If you do tell the cheater's spouse, you might be responsible for shattering her universe, and she will never thank you.
You do not know anything about her. You have never met her.
Do not meddle in other people's lives.
It is none of your business.
Should I expose someone who is cheating?
If you decide to expose your sister’s husband, are you ready to deal with the consequences? Typically, the only thing worse than a cheating spouse is someone who exposes a cheating spouse. Infidelity and affairs are very common (see stats about infidelity). And it is almost impossible to have an affair without someone knowing about it. But, most people keep quiet about it. When it comes to infidelity, most people believe that the best course of action is to “mind your own business.” And if you cross that line, rather than being seeing as a hero of the truth, you might be seen as a troublemaker (even by your own sister).
Should You Tell On A Cheater? I Wish Someone Had Told Me
There is an etiquette in these situations that I recommend. If you know the person being cheated on well enough, you should tell them in person, providing the details you know. Keep reminding them you are only telling them the truth because you honestly care and respect them; as sorry as you feel having to tell them, you would feel extremely sorry if their future was compromised by allowing the deception to continue.
However, the article continues...
In situations where you don't personally know the person, but have strong, first-hand knowledge, I encourage you to drop them a note, give them the details you do know. Tell them that you hope the information might explain suspicions they may already have and assure them that your only agenda is being honest. If they choose to ignore the information, at least you've done your due diligence.