The Other Husband by Kathryn Croft

One dinner party will destroy everything.
It's a perfect evening on the night that throws a wrecking ball into Abby's life. There's still a hint of summer sun in the purple-streaked sky. Abby and her best friend, Sienna, look on fondly as their two husbands laugh under the garden gazebo. None of them know it's the last time the four of them will be together again.
Bored of their daily routine and loosened up by wine, Abby finds herself going along with Sienna's plan for their husbands to swap homes for the night. 'How funny would it be – just for fun. Just to live a different life for a few hours. That's all!'
But what starts out as just a little fun, has devastating consequences for all four of them. Alone with Sienna's husband, Greg, Abby finds him unexpectedly charming. Inviting, even. And something happens that night that neither she nor Greg can take back.
Abby is desperate to tell her husband and Sienna the truth, but...