A divorce is what worked for me. My ex is now engaged to and has a kid with the coworker he swore he was just friends with.
They talked on the phone nearly every night after I went to work. My argument was that if she was just a friend, why did he keep her separate? We went out regularly with his other coworkers - dinner, bowling, movies, stuff like that - but with her, it was only him and her.
He says they were never physical while we were together, that they were just friends and he would talk to her about her boyfriend, work, etc and give her advice blah, blah, blah.
But the end result paints a much clearer picture. The fact is, physical or not, he fell in love with her because of how close they became.
I'm not at all saying your situation is the same or that you can't come to a different outcome, but you asked what worked for me. :)