So it's safe to say she's not thinking about how this impacts you at all right, she's just worried about what 3rd parties think of what she's doing -- she's worried about being embarrassed, it's all about her and not about you at all.
Therefore, you gain nothing by telling her how you found out. You just give her a reason to get angry at you and further justify what she is doing.
To the degree you will play "the bad guy" it makes it easier for her to rationalize what she's doing. Let her wonder.
^^^Bingo Acc
Me-45,W-36 M-12 yrs, T-15 years SS- 16 Nov 2003 Initial B date, 2-3 others since EA/PA OM 2003-2004 Reconciled 2004 May 2013 Final BD, W completely detaches W files D June 2013 I am moving out 26 July 2013