A poly person shouldn't be used as a fun diversion until the opportunity for a "real" relationship comes along. Or, at least, it should be said up front that that's what's happening. I suppose you can give her the benefit of the doubt and assume she didn't realize quite how she felt... but yeah, that's a shitty way to be broken up with.
I think you could beak up and stay friends. But I don't know about pretending your relationship never existed and staying friends. That desire to rewrite the past to smooth the way for the present, to put this new guy's potential feelings in every way over yours... that just seems kinda callous and cowardly. I would say no to that part.
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.