I accept that one can come into awareness that they are poly-wired later in life. You work it out with your peeps and talk things through then. Honestly.
You do not run off to have affairs. THEN start a conversation.
I could be wrong. But I'm under the impression she cheated, did NOT confess or ask forgiveness but was caught in a major relationship lie of omission. No intention of confessing til caught? Now suddenly wants to make it be about "going poly" rather than owning her own baggage in causing a mess in the rship with her cheat?
To me it isn't the THING that was done but the LIE that is the deal breaker.
How can anyone communicate well with a lying person? Too frustrating.