I am with Virginia 100%. I would not tell him anything. Just move. You do not have to justify anything to him. He did not tell you he was planning on seeking out a OW. He did not justify the OW to you either.
You have to become self absorbed. Think about you put you first. Do not worry about him and do not think about that OW. Don't worry about being "mean." Is he being "nice" to you? No. When you talk to him just be apathetic with a slight smile. Make him wonder what your thinking about.
Don't try to predict what he is going to think about you moving into your home office. Who cares. Your not doing this for him, you are doing it for your own sanity. It means nothing at this point. You still have till May to finish that project. Who knows, maybe with detaching you may not mind staying in business with him once May rolls around. I know you do not want her to share the fruits of the labor. However, she probably has her own job and her own fruits of labor. You need to think about the partnership and how it benefits you. If it benefits you economically to stay in the partnership then go for it. If it benefits you for job related stress and designation of responsibilities then go for it. You do not have to make these decisions today anyway. YOu still have time. Everything is still so fresh. See how it works out with the home office.
If he asks why you moved maybe just say "I can still get everything done from here so don't worry about it." Give a wink and walk away cause your too busy for anymore small talk.
