I’m a big believer in not doing couples therapy until the WS is at a point where they have come completely clean, and have spent some time working on themselves first.
There’s no point in paying someone else to hear your WSs lies. I never did couples counseling with my exWH, it just was never going to work. He never told me the complete truth, why would he tell a stranger the complete truth?
On the opposite end of that, when I was the OW, I had long conversations with the man I was involved with about his couples counseling sessions with his partner. Yes, that’s as awful as it sounds. I even drove him to his IC sessions quite often. So they were paying this person to help them, but instead of just lying to his partner, he was now also lying to this therapist. What a winner (and hey, I was a party to that sh*tshow). Even if your WS isn’t physically seeing the OW, even just talking to her while trying to “make things work” with you is a sign that things will never work. A relationship with someone who is also invested in someone else behind your back is a train wreck happening in slow motion
I think you’ll need to be very sure, and very certain that things have actually, and really truly ended between them before you even try. Couples therapy will not work if he’s still mentally coupled with someone else. I know you want to trust him and believe him, and I’m not suggesting you don’t, but I would trust, while verifying, at this point.