I debated the same thing, and while it felt good to let her know "I knew" it really didn't do much to help the M - if anything it created more friction between us and further dampened the chance for R as now it's one more hurdle she's got to get past and talk to me about (she'd have to do it anyways, but her knowing that I know puts reality to it as opposed to her potentially thinking she could hide it if she came back).
In retrospect I probably wish I had saved that conversation to a future date if she comes back.
On the other hand if you are going to talk to her - then you need to plan full exposure if she doesn't end it, which I'm not an advocate of.
Me: 40 W: 45 T: 13, M: 11 1 D: 9
Suspect A 6/15 ILBINILWY 8/15, and 3/16 EA/PA Discovered 3/16 EA admitted 3/16 W Moved out 4/16 W opens R talk and says A over 1/17