A 180 would be to start participating in their IM relationship. Ask him to email her a message for you. Somethign lighthearted. Keep doing this and acting like you enjoy the three-way IM fun until you take the illicit control thrill out of it for him. Do this for two or more months. Make him feel like a go-between between you two. Just a crazy thought of a totally different approach. Start off by sending both of them humorous email forwards or whatever. If you gave him permission, what would happen? Thrill might be gone.
