Maybe Dagferi shared their opinion a little strongly.
I'd agree with the sentiment though. People get to end relationships the way they want or need to. You get to feel anyway you want about how she's handling it, but I'd recommend giving her some space to cry, to have a last fuck or two, whatever. You don't need to take it personally if this guy you've never met doesn't feel comfortable being a "secondary."
Personally I feel your gf fucked up asking him to be her "secondary" after a few weeks of casual dating. It's kinda like telling someone you love them too soon, or asking them if you can call them bf/gf too soon. Only worse, because for a mono guy, there's an implied insult perhaps, in being thought of as secondary. Yuck. I'm as poly as can be, and I'd be insulted if someone asked me outright to be their secondary.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 60) loving miss pixi (poly, F, 38) since January 2009, living together since 2013
also loving Punk (monogamish, 42, M) since Oct 2015
"Master," (mono, 34), miss pixi's Dom for 2 years