You may want your mge., but right now your mge. is comprised of you, him, and your wife---You do not wnat to be part of a 3some.
You must be somewhat harsh about this---very calmly, w/out breaking down, and with an ice cold demeanor, tell her you know you cannot control what she does, but by the same token, you do control what you do
You give her NO CHOICES---she either ends her friendship, and goes completely NC, immediately, or you will end the mge.
Tell her she can go to him, tell her to pack all her belongings, and go, if that is what she wants. Tell her you will see an atty., immediately, and file for D.
Hopefully reality will set in, and she will see that what she is doing makes no sense, and will end it.
Your lack of relations with her, for the last however long, is more than likely due to the fact, that she is obsessing about him, and you are far back in 2nd place, he has been the big white elephant in your mge.
He is the cause of the crack in your mge.,---she may not get past this, and you may need to face the fact, that your mge., is over------
You need to bring her to the reality of all this, and force her to get off the fence, and decide which way she wants to go, either married to you, working toward R., or a life with someone, who she was unable to make it with in the 1st place
Also take all the marital assets/finances, and put them in an acct. with your name only on it---for now, also cancel all joint CC's----tell her if she wants any proceeds from the sale of the house---she needs to now pay her half of everything, starting immediately---that includes, car, all types of insurances, home expenses, and anything else
This hopefully will jerk her back to reality
If she decides to stay, have your boundaries ready, with actionable consequences.-------Also tell her she will need to sign a POST--NUP.
Do not argue with her, or discuss anything with her, do not raise your voice----say what you have to say, and leave---go run errands, go to work, whatever----and do not CONTACT HER, OR ANSWER HER----let her think about what her future will be like on her own, or with her lover, who she couldn't make it with before!!!!!!!