I have a hard time answering this question without getting a little "callback" joke to me walking out of my first MC's office.
Ask them if duty exists. Simple enough question, and very key in both infidelity and reconciliation. A question my first MC refused to answer after I explained that duty dictated why I did something.
Ask number two, "What can a person do to earn back trust?"
The MC needs to be the type that *will* actually hold the WS's feet to the fire and state plainly the having the affair is wrong, damaging, has killed all trust, and that recompense and rebuilding is necessary on the part of the WS.
The MC absolutely cannot be a "leave the socks around" therapist that treats an affair the same way as they treat a disagreement about one spouse leaving socks around the house wherever they happen to take them off. The actual conflict resolution or coping that you will get out of this type of MC is far too flexible. Emotional abuse is basically impossible for this type of MC to tell from a legitimate emotional desire.
Here's how this MC might sound
MC: "How does it feel when you see your partners socks lying around the house?"
Clean Spouse: "I feel like they don't respect the effort it takes to keep the house clean."
MC: "Dirty spouse, see how that makes CS feel?"
DS: "Well it's just a pair of socks, I'm so tired at the end of the day. To me it's not a big deal. I'm just trying to relax in my own home and I get a huge sense of relief to just take my shoes and socks off when I get home and plop down. I mean to pick them up but I forget sometimes is all."
MC: "CS, see, DS doesn't see this action as disrespectful, they are just trying to relax, do you think you could just pick up the socks and give a friendly reminder when you do instead of letting this get you really mad?"
MC: "How does the affair make you feel?"
BS: "Unsafe, hurt, and betrayed. Like my whole life has been taken away from me."
MC: "See WS, BS is hurt, doesn't that make you want to stop the affair?"
WS: "I think that love is complicated and you can love multiple people. I don't see why my love for someone else hurts BS."
MC: "BS, see, they don't mean to hurt you. That relationship is important to WS. Can you see past your insecurity to let WS continue a relationship with AP, and just remind them that you make them the most important part of your relationship when they step out on you?"
Don't be surprised if some bullshit very close to this happens.
They really don't care who capitulates. They just try to make each person see the other person's view. They don't pass judgment and they don't arbitrate. They try to get you to agree no matter the cost to either individual. It's the M they are out to save.