ok so it is an online affair.

Turn off the internet. Tell her you will not enable her EA.

You are allowed to state what makes you uncomfortable.

Or you get keyblaster or a program like that and install it and monitor her...

I used real spy monitor.....

Then when you have the intel.
Cut the internet off.

State the reason why. Do not give away your intel. Just state you know.
State your boundaries about an OM. OPEN MARRIAGE or OTHER MAN.
Then she has a choice.

It will be anger or remorse.
Then it will either push her towards leaving or not.
If its anger and leaving.
Then you expose. Very close friend who is a friend of the marriage to lean on her.
Then her parents and your parents.
IF the OM has a wife or GF let them know as well.

Read up on the carrot and the stick of Plan A.

Flowers always make people better, happier, and more helpful; they are sunshine, food and medicine for the soul.
unconditional love is awesome!