Moral dilemma - to expose a cheater or not?

I tried looking in the search bar for this topic but couldn’t find anything too relevant to what I’m posting. if this has been posted recently or previously I apologize.

if someone you know is being cheated on, do you inform them? or do you say “not my business not my place”?

most of the people in my life and that I’ve come across, including myself, have a general perspective that they would want to know if they were being cheated on.

it seems so simple, but really I find it’s not.

if it was your best friend would you tell them they are being cheated on?

if it was a family member? or what about a coworker? an acquaintance?

where does the line get drawn and why?

if so many say they would want to know, why is it that sadly, sometimes they find out from the cheater directly, but also others that knew and said nothing.

I ask because when I was 14, my dad drunkenly confessed to me one night that he cheated on my mom. he was regretful and said it was a mistake. I never told my mom, and I don’t think I ever will. they are still happily married to this day. though, I find myself questioning at times whether or not that is the right thing. if my best friend was being cheated on I wouldn’t hesitate to tell her and be there for her every step of the way.

conversely, when “the other woman/man” decides to out the cheater to their partner, is it the right thing to do? or is it up to the cheater always to come clean?