I would definitely tell him or her because t’s degrading, disrespectful and humiliating to not know and the one doing the cheating is likely carrying on like everything is good. The one being cheated on has a right to know so they can move on and possibly find someone with the same morals. I’ll personally never understand why people take vows with all this honor and cherish just to break every promise made. There’s plenty of people in the world who want that kind of open marriage so why crush someone’s heart who doesn’t. There’s no honor or respect in betrayal the bottom line is with out a doubt I would tell him or her.