Yer spouse is looking for someone who will listen to them without trying to control them. If you don't, they will turn to someone who will. Don't offer fixes or try to coerce your spouse into leaving their relationship with OM/OW. And never badmouth the OM/OW.
The idea is to create as large of a 'safety zone' for your spouse as possible. A zone where your spouse feels comfortable talking to you about anything. A zone in which you can further fight for your cause by displaying the changes you have made to yourself. This is much easier to do if you are on good speaking terms with your spouse.
Stay away from saying things that give the walkaway the impression that its OK for them to f*ck around on you with OM/OW. Things such as: "I hope you have a wonderful life with him(her)", If you love him(her), then maybe you should be with him(her)", "Do you love him(her)?", etc... Avoid using OM/OW's name when you speak to your spouse. Basically, speak to them like OM/OW does not exist. I guarantee your spouse will find it very odd that you totally look past OM/OW.
Things you can say: "I hope you find the happiness you are looking for", "Anyone would be lucky to have you", "I appreciate you being so open and honest with me".
Read about Divorce Busting« Telephone Coaching here!