S - I am very sorry to hear about the recent things you have discovered. IMO you confront, tell her what you know and move forward with a plan of action. If she thinks she is getting away with something her behavior will continue, she will get more bold and who knows where it will lead.
When my XW moved out and ultimately D'd me she told a mutual family friend one day that I should be thankful that she did what she did now versus waiting until our D's had graduated from high school and were on their own. I am projecting here obviously but I hope your W is not placating you and hiding in the shadows until your D has graduated and moved on. That's why I think you have to nip it in the bud ASAP.
My 10 cents....best of luck with whatever you decide.
I just don't want to make the wrong move too quickly. Things have been so great that I was shocked to see the messages. I can't over emphasize how great things have been.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018
