You say you're willing to consider breaking your moral code to avoid "potential damage". But that potential damage will only increase from what would likely be a temporary hurt if you stop now (Him: "You should have told me right away!") to what will likely be a devastating, scarring, relationship-ending hurt if you continue (Him: "You lied to me for how long, how often, about how many things?? You really don't respect me at all, do you... how can I ever trust you again?").
I urge you to look your motivation of avoiding potential damage square in the face and realize that you're looking at it in a self-serving way that will accomplish the opposite of your stated goal. If you actually want to give the two of them time to work this out, step back and give them that time, rather than fuck her behind his back and then expect him to realize that poly is a great idea once the truth comes to light... because it *will* come to light, it always does, sooner or later. All that lying will do is ensure this never works out.
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.