As long as he isn't going to think his OW is a "friend" and you are approving of his open marriage...
I would remove the argument part though, you don't want to show OW there is any fighting going on...
Re the marriage therapy... a marital therapist is a VERY DIFFERENT thing from a personal therapist... IC is a waste of time in repairing marital conflict in my opinion.
Don't wait for your H to "think about it"... just find a GOOD one and go individually to set an example.. it will take you a while to find a good one anyways. You want to "interview" them on your own first to be sure they are going to be valuable in helping you repair your marriage... MANY marriage therapists will tell your H useless things or even destructive things.. You want to filter those OUT before you convince him to go...
When a family member says they dont want to "take sides" or "get in the middle" you tell them this :
I am not asking you to choose me or him. I am asking you to choose our marriage over our divorce. I am asking you to speak up and support us in working with a family therapist to get through this rough point in our mariage. I am not asking you to attack my husband, I am asking you to protect him from making a mistake and protect our marriage from a dangerous interloper.
Oftentimes when you appeal to family for help they feel like you want them to attack your spouse and many are uncomfortable with that. Its important that you make it clear you love your spouse and want their help to motivate your H to work WITH you...
If your H brings up talking about your marriage with OW
Our marriage is none of HER business. She is NOT a licensed family therapist. If she was and doing what she's doing now she would have LOST her license a LONG TIME AGO. If you want to talk about your marriage you talk to a licensed objective trained professional... NOT someone you are having SEX with in SECRET.
So, just to be sure I understand things accurately, your H's position right now is that he's living with you, sharing a bed, having sex with you, but he wants a divorce?
Is that his position on the marriage right now?
Is your H accepting that he's having an emotional affair and claiming he's in love or is he just saying this woman is a "friend"?
