If she won't tell him then I will, and it's not going to be polite. I've let her know that if she can't separate from him outside of the occasional work encounter (they work for partnered marketing companies) then the BD is coming on the grounds of adultery.
Rev, you and I stared here about the same time, so I'm new too and can't offer much in the way of advice but you may want to ask yourself if the above is likely to get you closer to your goal and if it is DBing- detaching, stop talking about the A or the R, etc. You're not going to be able to force her to end the A, and ultimatums usually don't work (I've learned that one the hard way). Chilling out and backing off is hard, hard hard but confronting the OM is only likely to drive the W to him. I thought I was at the end of my rope too, and I told the W I want a separation- now I've discovered new found strength here and I wish I didn't. Don't do anything rash or that is going to drive her further away. Wishing you the best.
Me: 45 W:43 M: 15, T:21 2 Kids- S-14, D-12 A Started: 10/2013 Discovered as EA: 6/2014, as PA: 7/2014 A changing, not ending Start DB'ing 9/2014 Same house, same bed