Don't do this. Don't get mixed up in a relationship that isn't yours. Talk to the husband to talk with his wife and urge him to do so but don't tell her in his place. That isn't something you should decide for him and her. Tell him that you can't live with this secret as you care for her and you don't want her to live some kind of a lie. But that is the maximum that is up to you from my point of view.
Even if this isn't your ideal way of handling things, you can't dictate how others should lead their life.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.