I'm not offended. And actually I didn't think you were talking about the Iraq war, I just brought that in for my own reasons
If I saw a man beating up his child I would do everything in my power to stop him. I wouldn't just call the police. I would try to find a way to physically stop him.
Regarding your best friend and the cheating situation...
1. If everyone were following the golden rule your husband probably wouldn't be cheating unless both he and the woman he's cheating with are totally okay with someone cheating on them.
2. Yeah, your friend has to tell you to honor her own morals. Or maybe she tells your husband to tell you or she will. It's not her fault you wouldn't want to know. She's honoring her own morals by telling you the truth. Hey, at least someone is being honest with you.
3. If you wouldn't want to know your husband is cheating on you then perhaps you are tacitly giving him permission to cheat eh?
But seriously, it's not your friend's fault, the blame resides with your husband for putting the situation in motion. Don't blame the messenger.