Rather, I'd confront each of them separately. And don't cave in when they profess their innocence. I'd also tell her husband about your suspicions. He probably has several suspicions of his own. She certainly doesn't sound like a worthwhile friend to me.
Research suggests that happy couples are good communicators.
My hunch is your communication has broken down and neither one of you are sharing positive or negative feelings. Closeness and bonding grow with sharing thoughts, ideas, dreams and fears. And you certainly haven't shared your fears. You need to get started. Confront your husband and be prepared for some rough times with the arguments that may follow.