Personally, I can see why you would take that course of action. Ideally of course, if you had some REAL evidence to back it up, that would be perfect. Frankly, if you already know about the A, exposure and setting the attendant boundaries is a given.
FYI, I also held off exposing my W to OMW for roughly the same reasons you posted, so I kinda understand. It would have caused immense trauma to her personally and in the workplace. My sitch improved, but this point was a "neutral" one, I wouldn't say the non-exposure to OMW helped or obstructed the recovery process. But a big point is that, as long as it took, the fog did lift for my W. I cannot honestly tell you I would not have gone full blown into full exposure otherwise. But the wrong reaction by OMW at the wrong time would have hurt us on a few levels.
Sure, your W has to be responsible for what she is doing and its consequences. There's a line separating that from doing what's best to try and save the M if you that's the stand you are taking.
Me 42 W 39 Married: 11 Jan 1998, T: Since 1992 First Bomb: Sep 2007 Confirmed A/OM: 4 Nov 2007 Kids: D10, S5 Reconciled and together again after (alot of) time and heartbreak. 3rd kid, S, born 2 Jan 2010.