Her husband's response is not your responsibilty. He has a right to know; why should he be the only one of the four of you affected NOT to know the truth??
Stop making excuses.
Also, do NOT be specific in letting your husband know WHAT you know. It is probably THE single biggest tactical advantage you have : them not knowing what it is you know!
Say instead "I know all about you and (OW), and it needs to stop. It is incredibly disrespectful to me, to our marriage, and to our family, and it needs to stop." nif he demands to know whatcyou know, DON'T TELL HIM.
