Six weeks before Kyle Paxman’s scheduled wedding, a stranger walked into her office with e-mail messages and other evidence that Ms. Paxman’s fiancé was cheating on her — with the stranger’s girlfriend. “The dress had arrived, the flowers were done, the menus were chosen,” said Ms. Paxman, manager of two food and beverage outlets at La Costa Resort and Spa in Carlsbad, Calif. “One hundred and eighty guests had tickets from all over the country and the Virgin Islands to come and make a weekend of my wedding.” But rather than cancel the reception, planned for this Saturday in Vermont,...