Baseball
There are three reasons we are so hard on letting the OM GF know.
1)The moral stance. As betrayed people ourselves we all wish we had been told about the affair(s). If I could count the number of ex friends who told me in the first three months after my d-day that they knew but didn’t want to hurt me… And yes – the "ex" in front of the "friends" is intentional.
2)The preventive stance. Our collective experience is that once the OP spouse knows, the OP will focus on saving their relationship – often shoving the AP under the bus. We hate generalizations on this site, maybe especially gender-based, but MY experience indicates that your wife’s OM will make little of your wife – insisting she initiated, she was pressing him, she threatened to tell his gf, she ignored his pleas, that you are a nutcase, you are abusive… whatever. None of it matters – your goal has been reached.
Women on the other hand might turn to their AP once their partners are informed, but that’s when the truth about the "soulmate" comes to light: They get dismissed or dumped ASAP.
3)The clarity of seriousness. For your wife right now it was all just a "mistake" that impacted you and maybe her. But once she realizes how many her "fun" impacted, and the damage cost… it becomes serious.
Personally – I think 2) is enough to let her know. Just keep in mind that a message like "I think you deserve to know that my wife and your fiancé have been having a relationship that has gone too far" is enough. You don’t have to prove anything to her, your goal is to get her to question Loverboy and wake up the fear of God in him.
I can share from our collective experience that the fear you have is common, but the frequency of revenge from OM is extremely rare.
There hasn't been contact since February 15th
How do you know?
It’s not as if they would share contact with you.
I would suggest the following:
Tell her that the ONLY way forward is through the truth. That until you have a sense that you know EVERYTHING there is no way this marriage can ever be healthy, and you don’t want to be in anything but the best marriage ever.
Tell her that infidelity has made you question if there is any future for you two. If she tells you the absolute truth – even to the point of increasing your pain – there is hope, and it shows her commitment to rebuilding trust.
But at the same time, anything that you have to pull out of her, or discover through other sources and evidence will seriously erode at any hope you have to reconcile and will inevitably convince you that there is no future.
THAT includes you learning from any other source than HER if OM attempts any form of contact. Be that a poke on social media, a missed call, an e-mail, a joint friend asking…
Truth = a chance, falsehoods and lies = no chance.
"If, therefore, any be unhappy, let him remember that he is unhappy by reason of himself alone." Epictetus