Why not just say "Breaking up with [friend's name] at your request is eating me up to the point that I've been thinking about ending our marriage. I didn't realize it at the time, but this may be a dealbreaker for me. I know that me dating her was weird for you, but poly was weird to me at first and I adapted. If she'll have me back, are you willing to rescind the veto?"
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.