Look up on the net the "Hedge of Thorn" prayer, and pray it for your husband every day, printed if you can. It is a very powerful Christian prayer for our loved ones. I prayed it for my husband over thirty years ago. today he is a Christian and our marriage is very strong, thanks be to our Heavenly Father.
Dear Lord perform a miracle in this woman's marriage as you did to mine so many years ago. We know that when you fix something you fix it for good and better than anyone else can. Work in both her and her husband, and bring them to one accord as man and wife, remove that other woman out of this man's life, heart, mind or thought. Rekindle the love for his wife stronger than ever before, and may he allow you to guide his steps from now on, do the same for the wife. I pray this knowing you hear us and answer us better than we ever expect it to be, in Jesus's precious name amen.