Ah ha... so your instincts weren't so far off after all.
It's such a shame that your friend lied and made you out to be the bad guy. I get why she did it, but... not really cool at all.
The worrying thing now is that if/when you guys do come clean to your wife, she may feel 50 times more betrayed that you jointly kept this secret from her.
I know it's not up to you at this point -- you already did the hard work of being honest -- but I would strongly advocate for your friend to tell the truth about her lie, and why she did it, sooner rather than later. It will eventually come out somehow, and the longer it takes the more explode-y it may well be.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.