Also, I've gotta say... why do you even want to be romantically involved with someone who's perfectly willing to make a habit of lying to the face of someone she's supposedly deeply committed to, and who's a good partner to her? Why would you open yourself up to someone who's showing you that she's willing to expose her partners to emotional (being cheated on can really fuck people up) and physical (right now he thinks his STI risk is zero because he's in a mono relationship... except, nope, he actually can't make informed decisions about the level of risk he wants to accept, because he doesn't know who his partner is sleeping with) risks without their input? Doesn't exactly sound like a keeper to me.
I know you think you two have a special sort of honesty between you... but if she's willing to deceive him, then, buddy, sooner or later you're going to find out she's lying to you too... maybe about something really important.
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.