I normally completely agree with your advice, but I think with Anita and her husband's girlfriend, this information needs to come out. This could literally be a life or death, or at least life-changing, situation: if her husband gets an STD or even HIV from his girlfriend and passes it on to his wife, how then would you feel? I understand normally a person should just butt out of something this personal, but I think here it's imperative to intervene. Tell the husband: either you tell your wife or I will, because I won't be the one to let her develop herpes for the rest of her life or become sterile from the chlamydia you passed on to her or maybe even die from HPV-induced cancer. These aren't some random people, they are supposed to be your friends, and vice versa, so it seems selfish to "look the other way" and act as if nothing is happening.