That happened to my sister years ago. Her husband/my brother-in-law was secretly seeing someone else.
My sis didn't find out until the other woman called her to confirm that they were indeed separated, as he had said.
They separated after that, but I guess after a lot of marriage counselling and such, they got back together. Their second daughter was born just this past weekend. Honestly, I still hate the guy for what he did, but I can't deny that he's changed...a little. He's a pilot and flies all over the world, so I trust him about as far as I can throw him.