If you know about it, then it's no secret. Marriage records are public info. Why wouldn't your friend get a copy and take it to court? If this has been going on for four years now, there must be more to your story.
Her ex is not my friend, I have never met him, I too wonder why he does not check the records at city hall, if I was paying $36,000 per year, I darn well would check them regularly.
I think it is a pretty good secret, her 12 year old daughter does not even know about the marriage, my wife and I (and the couple) may very well be the only ones that know. Certainly as with any story there is more to it but I do not know what facts that I know to add.