Fedelia has a point. She must understand the concept of sharing. This is only my way, but here's what I'd do. I'd set up a time in a place your wife feels safe, and I'd set a table for three. Invite both wife and f, and though I'd keep the environment relaxed, I'd make sure the food was great. A real treat!
Then, I'd find some pictures or objects that are very special to you and your waif jointly, and some pix or objects that connect wife and F. Keep them near table but out of sight.
When I have both of them at the table, sitting beside each other with me(you) on the other side, and they are breathing in the fumes of awesome food they'd never want to run away from, I'd explain that I had something to talk about that was causeing me extreme pain.
Id first pull out the things that remind wife of me. Id touch on how much she IS and will ALWAYS beMy(your) love. Then pull out the stuff connecting her to F. Explain that you see and understand why your wife loves her so much. Leave these idems in front of them. Tell them that if they ever feel like doughting how much love is at the table for them, to just look at the objects. (I am right now just starting a poly situation with my love, and sometimes its hard, but I've known the woman he loves since she was 8. She is my companion. I love her in all but a physical way, though thats not bad either when my loves between us, she is amazing! I have always wanted her in my life anyway. It helps me to know that I can trust her implisantly. She would NEVER hurt me. If F truely is one of her best friends, then your wife can rest in the thought that F will always have her back, just like with the email.
Then tell them both how you feel. Tell them how much it hurts you to hide, but that it hurts to see them in pain also. Tell them everything, then LISTEN. Let them talk, to you, to each other. Use the food to keep them there. KEEP EATING. Have awesome desert. Show them how hard you'd work for their happiness (it's very hard to love two woman, my love has to dish out so much love and patience it's kind crazy).
I also think you should express to them that you'd be ready and able to do anything to at least talk things out and put things on the table for all to see. Tell them that they can say anything they want, and it won't anger you. Let them get emotional, angry, sad,whatever. You'll always be there!
Lastly, try to always say what you want, not what you don't want. Just sayin the words break up, unloved, fall out of love, etc. Even though you don't want these things, when a woman is upset, she will pick these out like buz words. Your just sayin them puts bad pix and possabilities in their heads.
whew! Sorry that was so long, but lead up and atmo are very important when deep feelings are "put on the table".
Much love to you, and if you take my advise or not, keep on loving. Be true, and humble.