Should I tell my friend her husband is cheating on her?

NO.  Your husband is correct.  Its none of your business and you should stay out of it.  Chances are very good the wife knows or suspects the cheating and its up to her to decide what to do about it.

I wouldn't do that because there's no telling how she will react.

Stay out of it. She probably already knows and will hate you for it. 

If she is your friend you should tell her. But do not bring your husbands name in the mix because that will stir things up in your relationship. You should find out where he goes with his mistress and bring his wife to confront him.

That is NONE of your business. Your only concern should be the bond between you and your husband. You will literally break y’all bond of trust and could cause him to lose his best friend. Not to mention create animosity between you and his best friend. You are that women friend not keeper. Trust me she already 

knows,  so all you will be doing is forcing her to face the truth that she clearly isn’t ready to accept yet. She can turn on you. Be a great wife and mind your business. Be there when she needs you. If it bothers you that much ask your husband to talk to his friend about disrespecting his union with his wife. If your husband continue to condone that behavior then question his morals too.

He has put you in a very difficult position, and you and/or your husband probably need to confront him about it. If nothing else, you need to be absolutely certain that it is true. He needs to know that his wife is your friend and that you are in two minds whether to tell her or not. Does he confide in his friends? If his wife was cheating on him, wouldn't he want to know? You could even ask his advice!

Your husband does not seem to share your values if he goes along with his friend. Nor does he appreciate how strong friendships between women can be - and that very often they are stronger friendships than men have. He needs to support you more than his friend: he made his marriage vows to you - not this friend.

Good Luck!

It's none of your business an not your place to tell his wife anything. If someone's going to wreck her world, why would you want to do it?  Let her husband do it -- she might already have some idea he's cheating anyway.  Women usually suspect or know. 

As her friend it is your business. If someone is doing harm to your friend how is it not your business? It would be your business even if she was a stranger to you.

What you know is gossip.   Your husband shouldn't have done that.

If your husband truly knows for a fact that he's having an affair and it's bothering his conscience, he needs to man up and tell his friend that he has 48 hours to tell his wife or he's going to do it for him.   It's better (IMO) for her to hear it from her own husband than to hear that hubby blabbed to best friend who blabbed, etc. and she's the last one in on the joke.

If/when wife needs your support, you can be there for her.   But it is improper for you to butt in and instigate the destruction of relationships based on gossip.    

Your husband is going to need your support too.   He's likely going to lose a friend, but IMO it's the right thing to do.

If your husband is unwilling to man up and handle this directly with his friend and would rather gossip and let someone else handle it, I'd be pretty disappointed in his moral character.

IF she is your friend, of course you should tell her.

Seems like maybe your husband should distance himself from this guy.  I would think that he would judge his friends by their character and this guy has none.